The town was infested with some sort of radioactivity from underground that was hurting people. I had to crawl underground, through these super-tight tunnels deep under the earth with things getting more and more evil as I went, until I was able to track the source of the radioactivity to a giant monster that lived back up on the surface (via the tunnels) in an abandoned barn. I had to fight the monster, but I could fly inside the barn. I ripped its head off, but every time I did, it was just like a big rubbery mask and a new head grew back, until I figured out how to do it fast enough that it wouldn’t have time to pop a new head out. When I did that, the real head came out: Tux the Linux Penguin.

That broke the immersion enough that I woke up, all amped up from adrenaline from fighting the monster.