Aspiring super-villain Dr. Horrible wants to join the Evil League of Evil and win the girl of his dreams, but his nemesis, Captain Hammer, stops him at every turn.

Into every generation a slayer is born: one girl in all the world, a chosen one. She alone will wield the strength and skill to fight the vampires, demons, and the forces of darkness; to stop the spread of their evil and the swell of their number. She is the Slayer.