For the first adventure, you don't want a case where they have to actually escape...if they escape on their own, they have no reason to listen to this "benefactor". My advice would be to have the first "adventure" be a single encounter, perhaps a bunch of bullies pick on the characters. (Or on some of them, and then the others come to their rescue; determine based on the characters' personalities whether they would in fact come to help.) After they beat off the bullies, the benefactor is impressed and offers them the deal. But if you want something more involved that can work too...make sure it doesn't end up with them not needing to take the offer, though.

As for the overall plot and the endgame, I'd say that that's really the essence of the campaign, and in order for it to be your campaign you have to decide on that. The best advice I can give is to forget for a moment it's a D&D campaign, and just write the outline for a story whose plot matches what you have so far and is of the right scale (e.g. 5th level characters shouldn't be fighting for the fate of the world), and then use that for your campaign.