I have a game I'm in the midst of preparing, so that I can DM once the current DM is over with his. The general plot is going to be vaguely a fantasy version of reign of fire.

Episode 1 (levels 6-8):
Start with a generic fantasy town. I'll probably just be modifying Fallcrest to suit my needs. Get the players invested in the town (as minor celebrities, chance of relationships, etc).

An npc asks them to get a certain dragonborn artifact, which they do (one of the early quests). He pays them.

The next day, they use their reward to pay somethign random: food, weapons, whatever. As they are paying for their items they see the gold (illusory) vanish before their eyes.

They approach the NPC (who owns a warehouse in the town). As they go in, they see him complete a final ritual, which happens to merge their current plane with a plane of dragons for a few minutes. A giant BBED (yes, D for Dragon) steps through.

NPC tries to intimidate, persuade dragon to work for him and promptly gets eaten (eats the artifact too). BBED flies off.

Now, they can pursue the dragon, run or fight. The town is beseiged by dragons and dragon like creatures (tiny drakes, all the way up to 2 full-fledged adult dragons, attackign different parts of the town). Here's the major test of my DMness. While the fight is going to be very difficult, I want the players to decide to stay back and defend their current home of their own free will.

Episode 2 (levels 12-15) and 3 (18-21)

The PCs kill off one dragon, other dragon gets driven off. In the next few seasons, years, the PCs stay to defend the town (explains the jump in levels). They fortify the town, and the town is now able to fend for themselves versus smaller threats, but still need the PCs to kill off the other dragon that ran away and who periodically drops in for raids.

Here's where it gets problematic. I kind of know the general plot. I want the players to:

1) Realise that the artifact is the key to removing all the dragons
2) Find out the location of the BBED (who has found a suitably epic lair, like the ruins of a major metropolis, or a dormant volcano, etc)
3) Find out a way to weaken the BBED (or we could have him be a lower level high paragon creature but that isn't as exciting)
4) Kill off the nearby dragon so that they can
5) Set off on their epic BBED slaying quest without needing to worry about the town.

But I don't know how to convince the players to do what I want them to do (while making it seem like it was their idea all along). It's going to be partly railroadey in the sense that I have a main arc and I want the players to go along with it, but I want to give the players the sense of freedom, and allow them to do their own things (within reason).

The reason why I 'fast forward' and power level the players between episodes and limit sandboxing is because. We're all working now and it's hard to complete any sort of campaign/story without people dropping out due to real life etc (6 players and 1 Dm, 2 of us are teachers, 3 of us have significant others, etc).

So... and ideas or plot hooks on what I should do?