The first thing we did (after finding a room to work in) was, of course, brainstorm. What would our game be about? The theme, "...And then it went horribly wrong," was pretty broad. We busted out a whiteboard and markers, and got a-thinkin'. We got a pretty broad range of ideas, from zombies to nuclear strikes to Clippy taking over the world to clone wars to everything else in between.
Finally, we landed on an idea that we liked: cannibalism. Think tribal island sort of stuff. But the more we talked about it, the more we liked one particular twist: you play as the cannibals. Nothing goes wrong for you; the island-goers' beach resort goes horribly wrong. Because they get eaten. By you, the cannibal.
A few minutes later, and we'd fleshed out a fuller premise. The cannibals' island was undiscovered until recently, when more civilized humans from the mainland discovered this tropical island with beautiful beaches. Fortunately for you, this means that delicious, unarmed humans are ripe for the picking! Your goal is to capture and eat the humans. But stealth is key -- if the humans notice that their island is inhabited by cannibals, then they'll stop sending ships your way!
Throughout the game you also pick up some curious items from the travelers you capture -- little bent pieces of metal. Paper clips, if you will. These are your currency. They are necessary for sacrificing at your temple: a laptop that your people found some years ago. The more paper clips you sacrifice to your god, the more your people are rewarded with various upgrades and abilities. The final upgrade allows you to summon your god, bringing him out of the etherspace of the laptop and onto the island to wreak havoc and exalt you forever more. Yes, your god, the great and marvelous Clippy, shall soon walk the earth.