"'Treehouse of Horror XX" is the fourth episode of Season 21. The episode first aired on October 18, 2009 on Fox and features three self-contained segments. An estimated 8.59 million viewers tuned in to watch the episode.
As various Gothic monsters Frankenstein's Monster, Count Dracula, a Mummy, and the Wolfman take a stroll around town on Halloween, they are bullied by Jimbo, Dolph, and Kearney for having "lame costumes". Deciding to buy actual costumes at a shop (Spongebob Squarepants, Iron Man, Captain Jack Sparrow from the Pirates of the Caribbean series and Harry Potter, respectively), they put them on and go over to the Simpson house. At the house they meet various women at the Halloween party, they begin flirting with them, and then suddenly the monsters' wives gatecrash the party and accuse that their husbands are cheating instead of going to kill children. When Homer then tries to convince the female monsters that their husbands weren't cheating because they were wearing costumes, the wives tear his head off and throw it into a bowl, having his eyes turn into the Roman numeral XX, which stands for 20.
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In a parody of Strangers on a Train, Lisa gets angry at Miss Hoover for not choosing her to represent the class at a reading roundup is is given detention. When in detention, she notices Bart is there too, upon which they concoct a plan together, during which Bart tells her that he'll get revenge on Miss Hoover if she gets revenge on Mrs. Krabappel for putting him in detention. Lisa ding-dong-ditches Mrs. Krabappel, but Bart kills Miss Hoover. When she wants to report Bart to Principal Skinner, she finds out Bart has killed him too, after which Lisa decides she has to kill Bart herself. When chasing him with a knife, they eventually get on a merry-go-round. Ultimately deciding not to kill him, Lisa throws her knife in the air instead, which then plummets back down, killing Bart. Lisa and Mrs. Krabappel then walk off, smiling at one another.
In a parody of 28 Days Later and Children of Men, Kent Brockman is reporting Krusty's new Burger², which is a new burger invented from cannibalistic cows. Despite this madness, everyone seems eager to eat the new burger. Kent is the first person to have a bite, and his eyes become red and pupil-less. He then starts becoming full of uncontrollable rage. He bites Krusty, who also becomes a Muncher. When a Muncher bites another person, that person also becomes infected with the virus and turns into another Muncher, giving them extremely pale skin and red eyes. Soon, the virus conquers Springfield with a vast majority of its inhabitants as Munchers. In response, the Simpsons completely boarded up their house completely; however, Bart ends up eating a Burger², without suffering any ill effects, leaving his parents puzzled.
Homer uses Bart's head to dial buttons on the phone to call Doctor Hibbert who is busy killing Munchers in his office. He says that it's possible that Bart is immune to the burgers' virus and instructs them to go to a "safe zone". When the family tries to sneak out of the house to take Bart to the safe zone, Homer exclaims loudly his relief for getting out of the house, attracting the Munchers. Despite their efforts to run for safety, the sheer number of Munchers is about to overcome them when Apu (who is uninfected due to his vegetarianism, and the fact that, as a store clerk, he has weapons stocked) arrives in an armored SUV, driving the family out of Springfield. However, the SUV gets stuck on a pile of junk at the city dump. Apu sends the vehicle on its way by pushing it down the pile, staying behind as he gets devoured. When the SUV runs out of fuel, the Simpsons are forced to travel the rest of the path on foot.
On the way, they notice Mr. Burns and Smithers lying unconscious by the road. Homer takes the opportunity to make fun of his boss, who springs to un-life and bites Homer's hand, infecting him as well. When Homer rejoins the family, Bart threatens to kill him, until Marge points out that he can be cured, once at the safe zone. Locking Homer inside a cage, they make it to the safe zone, wherein a stationed soldier says they need to eat Bart to gain his immunity. Not wanting to sacrifice her son, Marge advocates for an alternative.
Some time later, Lisa narrates that the world has returned to normal, as the survivors gained immunity to the Muncher plague and the surviving Munchers were cured, by taking soup with Bart's antibodies (something made possible due to putting him inside the cooking pot) Only Homer remains infected, seemingly because he refuses to eat food his son has been bathing in.
In a parody of Sweeney Todd: The Demon Barber of Fleet Street, in which is the style of a stage play, Homer and Marge are at Moe's Tavern together. Homer falls down the door that leads to the boiler room, and is impaled on a brewing machine. Moe doesn't want Marge to know that this happened, so he tells her that Homer wants to leave her to see what it's like to be gay. They drink beer together (which got mixed with Homer's blood). Marge decides to say yes, but later, Homer appears with what pumps the beer at the bar attached to him and attacks Moe. Even though Homer has a pump through his back Marge still loves Homer.
During the intermission, Kang and Kodos are within the massive audience. Kang says that this is the best play in lightyears, while Kodos reminds him that lightyears is a measure of distance.
At the end, in a parody of the show's closing sequence, everyone in the segment except for Kang and Kodos sing "Number XX". To the tune of the Simpsons Theme Song, the cast sings to the audience, telling them that they hope they enjoyed this years Halloween episode. Where Kang then shushes the audience, gesturing at Kodos, who has fallen asleep with the playbill over his helmet.
The episode was watched by 8.59 million viewers and was the most watched of "Animation Domination," which was ahead of American Dad! and The Cleveland Show.
IGN praised the episode by saying "it was a fine addition to the series' Halloween specials". The Sweeney Todd segment was "a nice treat". "Treehouse of Horror XX" won an award for "Best Writing in a Television Production" at the 37th Annie Awards.