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Tzekan
2007-01-14, 10:01 AM
If you play in my campaign (I don't think anyone who does in on the forums, but it pays to be careful)...get out of this thread!

Anyway, to everyone else, I've recently been GMing a GURPS campaign where the PC's travel between different realities and times (most vaguely scifi) doing their best to repair temporal damage that's been casued by an unknown disaster.

It's been very popular, and several people from my group have asked to join in, the trouble is i need some more inspiration for adventures which aren't a crucial part of the main story arc (otherwise it'd be over too soon for everyone). Here are some adventures that had taken place or soon will to give you an idea of what i've been doing, any adventure ideas would be make me a happy GM! (note, most of these aren't in exact order)

-The first space-craft to historically reach another world mysteriously explodes at launch (this was the opening adventure after the PC's met)
-A religious fanatic falls through a hole in reality to a primitive world, and uses his technology to act as a demonic god
-A "groundhog day" time loop occurs,because this can't be left out!
-Another allied group trying to repair the damage is captured by an aquatic race in the midst of a war and is being treated as enemy spies
-A team-mate disappears after a temporal shockwave rocks their home-time and reality
-A highly advanced (steampunk) culture is on the verge of developing temporal technology as well, and is unaware of the dangers...
-The PC's discover a seemingly ancient ruin ofa civilisation destroyed further back than they can travel, with writings speaking of a ritual which claims to be able to repair the timeline
-The home-reality of the PC's is devastated by an attack from a race which was registered as medieval only two years ago, but is now armed with weapons that can destroy planets and inpenetrable shields

That may seem like a lot, but we're all set on this campaign going on a LONG time. Thanks for any ideas in advance

Lord Sidereal
2007-01-14, 12:34 PM
time runs backwards at an accelrated rate for one character

a character must ward for his parents, who have been reverted back to toddlers

Thomas
2007-01-14, 12:39 PM
The plot from the Futurama episode, Time Keeps on Slippin'. Harlem Globetrotters optional.

Grey Watcher
2007-01-14, 02:00 PM
Past-nastification, anyone? (ie the Futurama episode Roswell That Ends Well)

You can also raid several episodes of the various Star Trek episodes and, of course, Doctor Who.

Heck, any sci-fi series is going to have at least SOME time travel, it's irresistable (Babylon 5 is the only one with time-travel plots that aren't as easily extractable).

Thomas
2007-01-14, 02:03 PM
Heck, any sci-fi series is going to have at least SOME time travel, it's irresistable (Babylon 5 is the only one with time-travel plots that aren't as easily extractable).

I don't know; when you strip them down, they're very general. I can totally see doing the Sinclair-Valen thing in FR, for instance - a character travels into the past and becomes this legendary hero that saved the world (and brings along a host of magic weapons, maybe - or a warband of high-level characters).

Jibar
2007-01-14, 02:08 PM
Past-nastification, anyone? (ie the Futurama episode Roswell That Ends Well)


Any player who tried that I would destroy.
Eeeewwww...

Tzekan
2007-01-14, 02:09 PM
You can also raid several episodes of the various Star Trek episodes


I think you mean the various Star Trek series...

But yeah, thanks for all your ideas so far, quite a lot of them are more light-hearted than mine, which is good because if its all doom and gloom my players won't really appreciate it as much when something really bad happens.

And curse mentioning Roswell, the temptation is so strong...must resist little green/grey men...

Roethke
2007-01-14, 02:24 PM
How 'bout something a la 12 Monkeys-- Cross-timeline infection?

The logic behind it would be that since viruses, microbes etc. are products of evolution (a chaotic process), the antibodies that people have are likely to be timeline specific.

Most of the time this might not be a problem, as the pathogens are more or less the same shape, (and whatever organization the PCs belong to that's sending them about probably takes this into consideration), but once in a while something slips through, with potentially devastating consequences. You can add some moral heft by making the PCs the carriers.

So an adventure could consist of
1) figuring out why timelines are being decimated by maybe similar looking diseases,
2) figuring out that it's the PCs (and maybe other NPC timeline travelers) who are spreading it.
3) figuring out who's doing it/ how to stop it. For an added twist it could be a upper-ranking mad-scientist type in the PC's organization who mucked about with the PC's immune system to make them carriers.

And just to make it more fun, one or more of the PCs (or a valued NPC) could actually get sick (with mild, but increasingly severe symptoms over the course of the adventure) just to add a little time-pressure to the whole jaunt.

~Roethke

Tzekan
2007-01-14, 03:03 PM
Roethke, i haven't seen 12 Monkeys but...wow, just wow

I'm getting all excited about this one. I've been thinking it's time for something huge to happen to the party, a trans-temporal outbreak fits the bill. Since one or two members of the party are from other times and dimensions, this could give them a very real threat (apart from main antagonists, who i don't plan to enter for a good while yet) to grapple with.

Disease is something our group hasn't really used before, they tend to think they can see what's coming...this should really get them unsuspecting, they need more fear and uncertainty.

Thanks again!

Green Bean
2007-01-14, 03:27 PM
How about an NPC who knows of way to fix the timeline, but vanishes. Your PCs learn that someone/something wants to keep the NPC quiet, and targets his/her ancestors. The PCs would have to go back in time to rescue this guy's grand parents to find out how to repair damage from the temporal disaster.

Merlin the Tuna
2007-01-14, 03:42 PM
Surely I'm not the only one that remembers Quantum Leap? The entire show was pretty much random sidequests through time. Wikipedia entry (http://en.wikipedia.org/wiki/Quantum_Leap_%28TV_series%29).

Matthew
2007-01-14, 08:32 PM
Past-nastification, anyone? (ie the Futurama episode Roswell That Ends Well).

"If causality doesn't care, then why the hell should I?"

Viscount Einstrauss
2007-01-14, 11:20 PM
I was planning this for a story of mine, but hey, you could try it out-

The villains out to wreck the timeline, especially anything the PC's try to fix, turn out to be the PC's from a future where changes they make end up destroying the world one day. Even better, try twisting them so that the PC's actually get to that point and go back to stop the temporal anomalies, and see if they recognize their situations as being the exact inverse of what they'd played previously.

TheOOB
2007-01-14, 11:37 PM
Going back to real world historical events is always fun. Fighting as a roman gladiator, storming normandy beach, holding back the confederates at gettysburg, fighting alongside alexander the great, all great stuff.

Dervag
2007-01-14, 11:52 PM
Not everybody enjoys being at a place where they know what will happen in history.

I would suggest setting up situations where the heroes think that they're completely screwed. Then something saves them- a spell scroll appears from nowhere, an antitank missile fired from concealment blows up the attacking robot, or whatever.

Then the heroes realize that it was future them who just saved their own bacon, and now have to figure out some elaborate process by which they can procure the item that they just saw themselves saved by. This works best with unique one-shot magic items- the heroes have to go on a quest to save their own lives.

Before you even start this campaign, I suggest that you give a lot of thought to how paradoxes work. Sometimes, the dice will not give the results you expect. Let's say that the heroes have gone back in time to fight alongside a legendary hero who saved the world. Then a fireball goes horribly awry, or somebody misses a shot badly, and the hero is killed before history records him saving the world.

Now what?

Mewtarthio
2007-01-15, 12:21 AM
Not everybody enjoys being at a place where they know what will happen in history.

I would suggest setting up situations where the heroes think that they're completely screwed. Then something saves them- a spell scroll appears from nowhere, an antitank missile fired from concealment blows up the attacking robot, or whatever.

Then the heroes realize that it was future them who just saved their own bacon, and now have to figure out some elaborate process by which they can procure the item that they just saw themselves saved by. This works best with unique one-shot magic items- the heroes have to go on a quest to save their own lives.

That doesn't make any sense. If the heroes get in a situation where death is inevitable, then they will die, and therefore they won't be able to save themselves in their personal futures, no? As such, the Deus Ex Machina shouldn't have been created to begin with, since they never would have made it to the future.

Now, what you could do is have the PCs be saved by help from an alternate future in which they do perish. Then, their actions result in whatever help they should have recieved becoming unavailable (say they kill their benefactor). That would force them to save themselves. I suppose you could also claim that the timeline they're in is the alternate of that alternate, but then things start to get confusing.


Before you even start this campaign, I suggest that you give a lot of thought to how paradoxes work. Sometimes, the dice will not give the results you expect. Let's say that the heroes have gone back in time to fight alongside a legendary hero who saved the world. Then a fireball goes horribly awry, or somebody misses a shot badly, and the hero is killed before history records him saving the world.

Now what?

Don't even get the dice out until the PCs "interfere" (that is to say, do an action that has an immediate observable effect on others; note of course that anything they do will eventually cause a ripple effect, but not enough to change much in the immediate future). Run the battle as history dictates it should run until the PCs step in (or get noticed). Essentially, you should treat past events prior to interference as though the PCs were watching a movie or having a story related to them, and you certainly don't roll in a story to see if the narrator made his saving throw (a la Prince of Persia: "No, no, that's not right; I didn't die. Let me start again").

Jade_Tarem
2007-01-15, 01:22 AM
One of the PC's (the smallest/largest/most intelligent/whatever) begins getting sick at certain points, inexplicably. The the other PC's eventually catch it (it becomes more preavalent as the game goes on) and realize it's whenever they're getting close to themselves in space and time. (Dragonrider(pern) series complex) - smart PC's will keep track and use it for hints as to where they went/go/have gone/will go/may go in the future/past/present/alternate reality (gets confusing, doesn't it?). This can have a bearing on their final solution. Or not. Your call. Really perceptive PC's can try to use it to trick a bad guy into meeting himself and getting blasted to atoms as reality compensates.

I'm using this one in my campaign. Briefly. One of the PC's gets separated from the group and whatever he or she does causes ripples big enough to affect the rest of the party - or vice versa. People disappear and things catch fire, etc, for the "future" group until they reunite.

You could even take one from Back to the Future, where Marty slowly begins to fade as he messes with his own past... although this was never explained. Still you could refine it. One of the PC's sometimes just... disappears. Then reappears later and acts like nothing has changed, including being knowledgeable about all the things that happened while he or she was gone. Something has happened in the past relative to the timeline the PC's are on and the vanishing PC's existence hangs in the balance. They have to find it as the disappearances become more frequent, and that's after they figure out what it is, before the PC vanishes completely.

I've always wanted to run something like this but never finished even the basics for a campaign so complex. My best wishes go to you, and a charge: to make at least one player's head explode before the end of the campaign. :P

Dervag
2007-01-15, 01:28 AM
That doesn't make any sense. If the heroes get in a situation where death is inevitable, then they will die, and therefore they won't be able to save themselves in their personal futures, no? As such, the Deus Ex Machina shouldn't have been created to begin with, since they never would have made it to the future.I think you misunderstood me.

The players find themselves in a situation where they THINK they are completely screwed. They've been cornered by a horde of enemies, or there's this big deadly thing charging down on them, or something like that. They aren't dead, but they fully expect to die very very soon.

Then, as a complete surprise to them, they are saved. A fireball torches most of the horde of enemies. Or a bazooka rocket blows up the big deadly thing. Or something like that. Someone just saved their lives.

Now that their lives are saved, they look over the battlefield and realize that they were saved by their own future selves, who traveled into the past to get them out of a jam. So now they have to go on a quest to make sure that they'll have the right weapon to save their own past selves- that scroll of fireball or that bazooka or whatever they used to save the day.

The point is that they don't die. They would have died, except for the intervention of time travelers. The time travelers happen to be future versions of themselves, so they have to then go on and figure out how to make the intervention work. There is no grandfather paradox involved here- the PCs do not die in the dangerous situation, even though they think they are going to die.


Now, what you could do is have the PCs be saved by help from an alternate future in which they do perish. Then, their actions result in whatever help they should have recieved becoming unavailable (say they kill their benefactor). That would force them to save themselves. I suppose you could also claim that the timeline they're in is the alternate of that alternate, but then things start to get confusing.Who needs alternate timelines?

My plot works even in a single timeline. Me-of-the-year X gets into a fight with a giant monster. The giant monster nearly kills me, but then it is destroyed by a blast of energy from the dreaded Flaz Gaz Heat Ray. I look around with relief and see that my savior looks just like me. My savior now vanishes into the time stream, possibly after telling me that I need to go find the Flaz Gaz Heat Ray and be ready to go back to the year X before it's too late.

I now set off on a quest to go find the Flaz Gaz Heat Ray. I presumably find it, go back to the year X, save past-me, and come back to my own time. Problem solved; no paradoxes or alternate timelines required.


Don't even get the dice out until the PCs "interfere" (that is to say, do an action that has an immediate observable effect on others; note of course that anything they do will eventually cause a ripple effect, but not enough to change much in the immediate future). Run the battle as history dictates it should run until the PCs step in (or get noticed). Essentially, you should treat past events prior to interference as though the PCs were watching a movie or having a story related to them, and you certainly don't roll in a story to see if the narrator made his saving throw (a la Prince of Persia: "No, no, that's not right; I didn't die. Let me start again").Once again, I think you misunderstood what I was trying to say. By a big margin.

I mean that the heroes can go back into the past and mess history up. They might go back in time and try to save Jesus with a submachine gun. They might try to fight alongside Alexander the Great and accidentally distract him and get him killed. Or they might convince Abraham Lincoln not to go to the theater on that final night of his life. Some things the heroes do might change the course of history.

If they do those things, what does the DM do? Does he fudge the dice like crazy so that history proceeds unchanged? Does he spawn an 'alternate timeline' in which the PCs' actions can exist without messing up the history of the PCs' own timeline? Or does he retcon the PCs' timeline somehow, so as to reflect the dramatic chcanges in the course of history caused by the heroes' actions?

Option (1) is my favorite because I believe in the Law of Conservation of History: you can't alter events in your own causal past because they have already happened to you. I can't unsay what I said to Sally yesterday. Nor can I unsay what I will say to Sally a week from now using information gained from time travel two weeks into the future- if what I say to Sally is part of my causal past, then I can't alter it. I can cause it to happen by mistake a la Oedipus Rex. But I can't make it NOT happen.

Were-Sandwich
2007-01-15, 03:35 PM
Temporal campaigns are good in that its easy to swap out characters. Take a que from Andromeda. If a player is bored of their character, an alternate timeline version of them pops up, and they have to change places. Skin colour change optional.

Tzekan
2007-01-15, 04:13 PM
Well, i picked a temporal campaign since it has so much potential for unique stroylines and i could it going a while, but the amount of ideas people are coming up with is amazing. I'm really grateful.

The swapping characters ideas is interesting, i'll have to think whether it suits what's happening and the mood of the campaign (can be quite dark sometimes) but it's certainly a consideration, as is everyone else's contribution

Mewtarthio
2007-01-15, 05:09 PM
I think you misunderstood me.

The players find themselves in a situation where they THINK they are completely screwed. They've been cornered by a horde of enemies, or there's this big deadly thing charging down on them, or something like that. They aren't dead, but they fully expect to die very very soon.

Then, as a complete surprise to them, they are saved. A fireball torches most of the horde of enemies. Or a bazooka rocket blows up the big deadly thing. Or something like that. Someone just saved their lives.

Now that their lives are saved, they look over the battlefield and realize that they were saved by their own future selves, who traveled into the past to get them out of a jam. So now they have to go on a quest to make sure that they'll have the right weapon to save their own past selves- that scroll of fireball or that bazooka or whatever they used to save the day.

The point is that they don't die. They would have died, except for the intervention of time travelers. The time travelers happen to be future versions of themselves, so they have to then go on and figure out how to make the intervention work. There is no grandfather paradox involved here- the PCs do not die in the dangerous situation, even though they think they are going to die.

I'm not arguing a Grandfather paradox. I'm saying that the heroes can't save themselves from the future because, if their own intervention is their only hope, then they won't survive in the first place. Look at it this way: Let's say I start a time travel experiment in which I send a message back in time one hour. One hour prior to the experiment, a slip of paper with the message on it appears. I take the paper, and use it in the experiment. This example, I believe, is impossible, because the slip of paper should never have existed in the first place.


Who needs alternate timelines?

My plot works even in a single timeline. Me-of-the-year X gets into a fight with a giant monster. The giant monster nearly kills me, but then it is destroyed by a blast of energy from the dreaded Flaz Gaz Heat Ray. I look around with relief and see that my savior looks just like me. My savior now vanishes into the time stream, possibly after telling me that I need to go find the Flaz Gaz Heat Ray and be ready to go back to the year X before it's too late.

I now set off on a quest to go find the Flaz Gaz Heat Ray. I presumably find it, go back to the year X, save past-me, and come back to my own time. Problem solved; no paradoxes or alternate timelines required.

I might have used the term "alternate timeline" in a confusing manner. I didn't mean it in the common definition of "timeline that runs parallel to our own." I simply meant a timeline post-interference. An "alternate" version of history in which the time travellers have interfered with the past.


Once again, I think you misunderstood what I was trying to say. By a big margin.

I mean that the heroes can go back into the past and mess history up. They might go back in time and try to save Jesus with a submachine gun. They might try to fight alongside Alexander the Great and accidentally distract him and get him killed. Or they might convince Abraham Lincoln not to go to the theater on that final night of his life. Some things the heroes do might change the course of history.

If they do those things, what does the DM do? Does he fudge the dice like crazy so that history proceeds unchanged? Does he spawn an 'alternate timeline' in which the PCs' actions can exist without messing up the history of the PCs' own timeline? Or does he retcon the PCs' timeline somehow, so as to reflect the dramatic chcanges in the course of history caused by the heroes' actions?

Option (1) is my favorite because I believe in the Law of Conservation of History: you can't alter events in your own causal past because they have already happened to you. I can't unsay what I said to Sally yesterday. Nor can I unsay what I will say to Sally a week from now using information gained from time travel two weeks into the future- if what I say to Sally is part of my causal past, then I can't alter it. I can cause it to happen by mistake a la Oedipus Rex. But I can't make it NOT happen.

You may not be able to alter events in your own causal past, but once you're time-travelling, your own personal past becomes quite different from the past of history. Let's take the famous Grandfather Paradox. You travel back in time and kill your own grandfather. Popular theory claims that this causes a paradox, because you can't have existed in the first place. I, however, believe that you do exist. Your own causal past includes the birth of your parents and the survival of your grandfather. You already exist in the current timeline. The fact that, historically, you will never be born changes nothing. You personally remember being born, and that's all that matters. From the perspective of the current timeline, you suddenly popped into existence with all the memories of a denzien of the future and killed some random guy on the street.

As for altering historical events: You're right, I did misunderstand. I still think that the best way to deal with this is the following:

Determine whether or not the timeline can easily be corrected. Say someone kills Hitler as a baby. It's quite simple to fix: Another charismatic German officer grows up and commits exactly the same actions in exactly the same manner (or virtually the same manner, anyway: At least, close enough that history is unchanged. You may tweak a few details, like having him be executed at Nuremburg or something if you want a bit of flavor).

Technically, the Cascade effect (which I believe in) would make any alteration to the past have drastic reprecussions, rendering the future unrecognizable (the only difference among levels of interference being how long it takes everything to unravel), but you should probably leave this out or everything gets too complex. Instead, do the reverse: The only difference between levels of interference is how long it takes the timeline to get back to normal. If the traveller does something minor, like killing his own grandfather, the effect is trivial and things soon go back to normal. Something a bit more substantial like assassinating a public figure may take years or even decades to correct. A massive thing like preventing or starting a war leaves its mark for centuries. A truly drastic thing like setting up a future tech dictatorship may change everything, though (or at least leave such a huge mark that the timeline isn't repaired until it approaches the entropic destruction of the universe).

As a believer in the Cascade Effect, of course, I find this illogical, but you really don't want to try and simulate the Cascade Effect. Trust me, people will appreciate it if changes end up being very minor in the long run (so they can actually go to the university they grew up in and see their own busts, rather than go to the university they grew up in and see Overlord Karglast's humanburger deluxe restaurant/clothing store).

Taedirk
2007-01-15, 05:45 PM
I'm not arguing a Grandfather paradox. I'm saying that the heroes can't save themselves from the future because, if their own intervention is their only hope, then they won't survive in the first place. Look at it this way: Let's say I start a time travel experiment in which I send a message back in time one hour. One hour prior to the experiment, a slip of paper with the message on it appears. I take the paper, and use it in the experiment. This example, I believe, is impossible, because the slip of paper should never have existed in the first place.

I believe this is an Ontological paradox (http://en.wikipedia.org/wiki/Ontological_paradox). If I perceive it correctly, it works with the idea that the time line is immutable and whatever has/is/will happen has/is going to/will happen. Take a look at the entry for By His Bootstraps (http://en.wikipedia.org/wiki/By_His_Bootstraps) and see if it makes any more sense that way. The whole ontological paradox thing seems like it's just another name to predestination paradox with a bit of Fate thrown in. Of course, the whole thing hinges on how you view time travel, alternate time lines, and all that jazz.

Yakk
2007-01-15, 06:50 PM
Interdimensional trade. They find a civilization that has resources the present wants, so attempt to set up a trading post. Something goes wrong, and the group is asked to do a task for the host kingdom in exchange for the foulup.

--

A second group of time travellers, working at cross purposes, is glimpsed.

--

Escorting a group of academics who want to record an event. Everything goes fine, but when they return, one doesn't come back. Nobody else in the future remembers him going with the party.

--

Spy mission. Someone was sent on a deep cover mission 10 years ago. They signal for help. The group is sent back using things that look low-tech to find the undercover agent and retrieve them. Gross changes are to be avoided.

--

Sabotage mission. Landing on the moon of Pluto contaminated the life that existed there before. It was nearly wiped out in the current timeline, and unreplaceable science was lost. In an alternate timeline, your team is sent back to help the terrorists who tried to stop the launch. It won't be viable to send another probe for centuries, because of planitary alignment...

--

They find an alternate earth with FTL and aliens present. You are sent to investigate.

Phase 1: Get samples of technology.
Phase 2: Get data on where the aliens came from.
Phase 3: Kidnap and/or persuade some humans in order to get their techological knowledge.

It is considered of HIGHEST importance that the other world doesn't get time travel tech, or even the possibility of it's existance.

--

Zoo capture mission. Your job is to bring a living ancient animal back. Maybe even a mythical one.

--

Martians! It turns out that Mars was inhabitted until about 1000 AD. At that point, runaway nanotech wiped out almost all evidence of the civilization. A pre-1000 AD observatory is built to get information on the aliens.

--

Gamma World. A world inhabitted by strange mutated humans and animals, together with a large machine intelligence. The machine intelligence produces the mutants in an attempt to populate the world.

--

NAZIS! Some Nazis in a world where they won WWII get ahold of a temporal agent who knows just a bit too much about time travel. A subprefect of the empire is trying to build a time travel device, and they are getting uncomfortably close. Your agency has decided to arm the opposition, and you are going along as advisors.

--

You visit what looks like a pretty standard 2000 AD world on a pretty standard job, and all of a sudden technology stops working. Guns don't work, electronics don't work, internal combustion engines don't work, electrical power doesn't work. Civilization starts collapsing very very quickly. Fires run out of control, civilization starts falling apart. Food transportation doesn't work, food harvesting doesn't work, and cities are too densely populated for their local food supplies -- every gram of food within 10 days walk of a city isn't enough to feed the population of the city travelling for 10 days walking.

There are blind pickup planned at a particular location at a particular time -- ie, anyone at that location, in the middle of nowhere, will be picked up remotely. You can only hope that the pickup still works.

--

Segway with different characters: Your characters are in a very beaten down version of their "present". They are brought in to a briefing.

Humanity is dieing. The last viable baby was 30 years ago. They have reason to believe that we can fix it.

Their goal is to stop the Wet Firecracker war, which destroyed human civilization 10,000 years ago in the aftermath of the "Cuban Missile Crisis". They have strong evidence that that wasn't the original timeline, and that some other time travellers messed things up. Your job is to go back and fix it, even though this will result in the annihilation of this entire timeline.

JadedDM
2007-01-15, 07:15 PM
Quantum Leap is my favorite show ever.

You can use the "Evil Leaper" storyline pretty easily. The party is assigned to various points in time to fix things and 'right wrongs'. (Assuming they are of good alignment). However, they then come to learn that another party, an evil one, is also hopping around in time trying to re-wrong the rights they just did! (Wow, that sentence is confusing.)

Jade_Tarem
2007-01-16, 03:20 AM
The Ontological paradox can be mostly resolved with the spirit of the thing intact if you're willing to involve slightly more than the PCs. The thing is, Dervag, that Mewthario's point is valid. It's the classic "motivation" thing... If you fix everything in the past you have no motivation to do it in the present, so it doesn't get done and nothing is fixed... so you do have the motivation to go back... etc, etc. It's the same thing only he's arguing the "how did it start?" angle, namely that there's theoretically a version of the PCs somewhere that is the "first" group that would have no help from a previous group - causing an abrupt end to the time loop.

There is a way out though, if you're willing to conveniently ignore the universal ramifications of the changes made. The example I'm going to use is from The White Dragon from Anne McCaffrey, where the queen egg is stolen from Ramoth's clutch but is mysteriously returned minutes later. Everyone knows who stole it but have no way of tracking them through time, and assume that one of them just had a change of heart. The problem is that Jaxom and his dragon, Ruth, are repeatedly haunted by visions sent to them by the fire lizards, an animal race with a collective memory. - They remember that Ruth "stole" the egg, which isn't correct in the literal sense. Turns out that he stole it from the thieves, and was the mysterious returner. So he had the motivation to do some time travel outside the bounds of what the characters knew normally.

The problem with using that off the shelf is that Jaxom and Ruth were initially still alive and able to be motivated after the theft of the egg. This isn't the case with your PCs. So you need a third agent to be the "first" savior. He can then convince the first PCs that they need to go back in time to save themselves and start the crazy little loop in the time line. That's just your backup story, though. If the PCs never question it then you can skip all that, but if they do then you have an answer, assuming they want to go all the way back in time and see.