Game tonight and a player can't make it. Help plz.

[Sir cryosin]

So I run a game for 3 players. Right now there at level 3 or 5. I'm running a sci-fi fantasy game and they took on a bounty to find this drug dealer that did not hold up his deal with a Mafia Boss. Boss put a bounty out for him bring back alive. The party has followed Clues tracked this guy down to the sewer systems and last session had a slight a small little fight with him. They managed to knock him out but they ended up cutting both his hands off. Now what I have planned was for them to be. Ambushed by another mercenary group that they have that they have some back history with. At a game they don't know they don't know these guys what their characters in game has ran into this other group.

So I plan to have this big encounter with this other mercenary group and with the three players. But I don't want to have this encounter without my three players at the table. Now I'm a DM that about 90%of my stuff is created right there at the table. I just come up with a base idea of where it could go and a few people here and there and what not. Anyway I have no ideas what to do.

I'm looking for ideas to keep the party inside the sewers 4 a session.

[Tiamatwing]

You could just go with the simplest option, postpone the game a week if the encounter is that important.
If you want to run next session, have them run into an advance party of mooks/search robots, or a wandering monster because sewers and who knows what is down there (giant croc/defective robot etc)

[DeTess]

Now what I have planned was for them to be. Ambushed by another mercenary group that they have that they have some back history with. At a game they don't know they don't know these guys what their characters in game has ran into this other group.

What exactly are you trying to say here?

Anyway, If I understand the issue correctly, you have an important story event planned for the game, but one of your players can't make it, and it is important all players are there for the event.
First of all, does this event absolutely have to happen in the sewers/before they hand their bounty in? If that is not the case, then you could continue out of the sewers, have the PC's hand the bounty in, and then fill up the rest of the session with a smaller side-story (maybe another bounty, or something from the back-story of one of the two attending PC's). The reason I'm suggesting this first is that many methods of delaying them and keeping them in the sewers for a session could feel forced/unnatural.

If the ambush has to happen in the sewers, but the PC's handing in the bounty isn't an issue, then you could just let them leave, and then send them back in at the end of the session, with a new bounty, or some other reason.

If you absolutely need them to remain in the sewers, there's a couple of ways you could handle it. If the PC's don't have a map, you could have the part they entered from collapse, forcing them to find an alternate exit, with all the relevant encounters that would involve. Alternatively, you could have some third party kidnap the player that won't be attending, forcing the remaining two to rescue him before leaving.

[Corran]

Does your setting have illusion magic and/or dieties? Or since it is sci-fi, halucinogen sleeping gases?

If it does, here is a somewhat cliche way to go about it.
The players fall into a trap that sprays them with a gas that nearly knocks them unconscious (or so you tell the players, rolling some damage perhaps for the eyes of the world, since in reality their characters are indeed knocked unconscious and they are ''dreaming'' what they play. Then play the session. Perhaps even ''kill'' their characters at the end of the session, which is what awakes them and brings them back to their senses (back in the sewer), in a state of agony.

You could have this dream sequence be whatever you want, since it wont really matter after the players ''wake up''. You could even have it be prophetic if you want to, like some greater power (diety? all-powerful illusionist? whathaveyou) wants to warn them about what is going to happen (what you have planned). Or to just try to manipulate them showing them false warnings. In that last case it could just be a magic trap of sorts.

What they played, could just have been just one of the character's ''dream'', and before the next session you narrate the other players what happened in their ''dreams'' (enter plot hook).

Is their prisoner still with them when they awake? Or did he return to consciousness first and fleed? (I dont suggest the last one, as this will all seem like a cheap way for you to screw them, but you could still have the enemy not be there when they wake up just for them to panic for a while, but they catch after him and they recapture him shortafter).

[Sir cryosin]

It doesn't have to be in the sewers. I'm just liking to postpone the main story just a bit.

What I'm thinking is having the drug addict escape and they have to track him down again but this time way easier.

Having them run into some creatures in the sewer. Last time they ran into a couple of corpse's fill with rot grub's. Don't want to use that again but maybe a group of junkies on a necromantic drug.

Maybe a skill challenge dealing with the pipes flooding.

Ohh i got it the bounty wakes up before they get out escapes. And as they are chasing him they hear the roaring of rushing water. They go through the flooding pipes skill challenge. They wash up in a area of dead junkies. That are undead thanks to necromantic drug.

[xanderh]

Personally, I usually just post-pone the game one week whenever this happens. If you and your players want, you can always do something else that day. Run a one-shot, or play a board game.

[ZorroGames]

Personally, I usually just post-pone the game one week whenever this happens. If you and your players want, you can always do something else that day. Run a one-shot, or play a board game.

This does sound the easiest.

[Honest Tiefling]

1) Bait and Switch. Sure, the mercenaries are there, but something went wrong with their ambush. The party gets tipped off, they were attacked by a rival group, or just plain bad luck. This isn't the climax, but rather foreshadowing of it as the group deals with the faux-ambush. And then plan an even nastier ambush to strike when the player's aren't expecting it or weakened later.

2) Another Bait and Switch. The mercenary group appears, but not before they sent in some small-time gangsters to soften up the PCs. Interrogating one might reveal answers, or they could just shoot them.

3) Tell others the situation, and send the situation digging through the Drug Dealer's stuff and doing investigation type things. There's loot and info there to get more loot, two of a PC's favorite things. Set up some hooks for later in the plot. He might have set up some caches, so have them running around the sewers after the potential loot.