Something Rotten is a humorous fiction about a reality that shifts among absurdities in real life, fiction, and the space between the living and dead. The fictional reality known as Book World contains both characters from stories and living human beings from real life. The fictional characters can also enter real life and interact with humans. Crazy time travel plays another important role in the story.

Thursday Next narrates most of her story in which she has returned to real life from a mission she had to accomplish in Book World. Her husband, Landen, has been eradicated via a time travel trick that involved interrupting his moment of conception. In this timeline, Landen had never been born. Thursday visits her mother and brother, Joffy, and her two-year-old son, Friday. Upon learning that an evil dictator from fiction, Yorrick Kaine, is trying to take over England, Thursday determines that he must be put back into fiction by reading the story in which he appears to him. The trouble is that nobody knows what story that might be. However, Thursday has other problems. Her bank account is seriously overdrawn, and she needs to get her old job back as SpecOps operative. She also has an assassin trying to kill her and a father who keeps stopping time when he shows up. He is with SpecOps as a ChronoGuard, a man who tries to keep time travel from getting too insane.

Her father tells Thursday that she must somehow win the SuperHoop croquet game, but the Swindon Mallets play badly. If she does not manage to win the game, nuclear war will destroy the earth. A key to all this is a St. Zvlkx from the Thirteenth Century who makes predictions called revealments. He becomes reincarnated and has his Book of Revealments with him. Meanwhile, the Goliath Corporation is trying to become a world religion in order to control all politics, commerce, and thought. Thursday's plate becomes full as she must do the impossible in order to save the world. She has to win the SuperHoop croquet game.

The Goliath Corporation and Kaine work against her. She has recruited some cloned Neanderthals to help win the game, but the rules disallow them. The Swindon Mallets are losing badly when a lawyer for the team discovers that the Neanderthals can play after all. By the end of the last third of the game, the teams are tied and must go into sudden death overtime. Thursday is shot in the head and almost dies but shecomes back to life from working a deal with her assassin. The Swindon Mallets win the game, thereby saving the world. The Book of Revealments turns out to be a record of bets that St. Zvlkx had made in the Thirteenth Century, and now are worth enough to buy out the Goliath Corporation. By the end of the story, the world is saved. The Goliath Corporation is reformed and Yorrick Kaine becomes a real human being but entirely harmless. Thursday learns a great deal about her future life.