"Treehouse of Horror XX" is the fourth episode of the twenty-first season of the American animated television series The Simpsons.

This is the twentieth "Treehouse of Horror" installment, containing three self-contained stories: In "Dial 'M' for Murder or Press '#' to Return to Main Menu", Lisa is forced into a Hitchcock-esque murder scheme by Bart; in "Don't Have a Cow, Mankind", Springfield is once again overrun by zombies thanks to Krusty Burger's latest sandwich; and in the Sweeney Todd parody, "There's No Business Like Moe Business", Moe the Bartender bleeds Homer dry to create the perfect microbrewed beer.

[1] Traditional horror movie characters such as Frankenstein's Monster, the Mummy, the Wolf Man, and Dracula roam the streets of Springfield on Halloween night.

They then go to an adult Halloween party thrown by Homer and Marge and hook up with the single women there, but are soon confronted by their wives, who are angry that their husbands lied about going out to kill children tonight.

Homer tries to intervene as the wives begin to assault their husbands, only to be decapitated, and his eyes are then replaced with two red x's, the title of the episode then appears onscreen.

[2] In this black-and-white segment and a parody of Alfred Hitchcock's films, Lisa wants revenge after being sent to detention by Ms. Hoover for disrupting class as she claims another student topped her through a tutor.

Bart, who also got sent to detention per usual, hatches a "criss-cross" plan and tells Lisa that if she pulls a "ding-dong ditch" on Mrs. Krabappel, he will do the same to Ms. Hoover.

Bart finally finds shelter at a crowded playground carousel, but is left at Lisa's mercy when the other children leave to get ice cream.

Upon eating a burger on live television, Kent Brockman begins to comment on the taste before turning gray, red-eyed, and cannibalistic, starting a chain reaction of people getting attacked and eaten.

Apu explains that he was never infected because, as a vegetarian, he did not eat any of the tainted beef, and as a convenience store owner, he is "armed to the teeth" (with a large arsenal of military-grade firearms).

After Apu is killed by the munchers, the Simpsons drive on but run out of gas in the middle of the highway, walking the remaining 20 miles to the safe zone.

In a parody of Sweeney Todd: The Demon Barber of Fleet Street, presented as a stage musical, Moe is lonely and upset that he does not have a girlfriend and is jealous when he sees Homer and Marge together.

In the opening scene of the episode, the monsters respectively wear costumes of Iron Man, Harry Potter, SpongeBob SquarePants and Jack Sparrow.