The premise? The four PCs are criminals. They are taken atop Mount Grimrock to a dungeon so remote it must be reached by airship. They are absolved of their crimes as soon as the guards throw them into the pit that leads into the ancient dungeons underneath Mount Grimrock. As such, they are free if they escape, but since no one has ever survived the deathtrap that is Mount Grimrock, their odds of survival seem slim.
That's it. They start with no equipment, and have to stagger their way through the dungeon.
Personally, that's what drew me — I fell in love with the premise, and love the idea of a megadungeon used as an actual prison (despite the name, fantasy dungeons tend to have another connotation rather than historical dungeons). It'd be a neat premise for something such as, say, a World's Largest Dungeon campaign.
Anyway, the Legend of Grimrock has a website, or you can just buy it off GOG.com or Steam, or you can read about it on Wikipedia (plot spoilers ahoy!), or you can read a review on Something Awful.