"'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.
In "Dial 'M' for Murder or Press Main Menu," Lisa is forced into a Hitchcockian 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 "There's No Business like Moe Business," Moe the bartender bleeds Homer dry to create the perfect microbrewed beer.
As various Gothic monsters, including Frankenstein's Monster, Count Dracula, a Mummy, and the Wolfman take a stroll around town at Halloween, they are bullied by Jimbo, Dolph, and Kearney for having "lame costumes". Deciding to buy actual costumes at a shop, 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 their husbands of cheating. When Homer then tries to convince the female monsters that their husbands weren't cheating because they were wearing costumes, his head gets torn off and thrown into a bowl, having his eyes turn into the Roman numeral XX.
'M' for Murder (or Press # to Return to Main Menu)Edit
After getting mad at Miss Hoover for not choosing her to represent the class at a reading roundup, Lisa 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. This is a alfred hitchcock spoof.
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. Twenty-eight days later, the virus has conquered Springfield with a vast majority of its inhabitants as Munchers.
The Simpsons have boarded up their house completely, and Homer won't even let his father in. Bart is hungry but would apparently rather starve than eat his family's fruit, so he sneaks out and against all common sense eats a Burger². When Bart returns and Marge discovers that he has eaten a burger, Homer goes mad and attempts to beat him to death, but Bart replies that the burger did not change him into a Muncher - he is immune to the disease. Homer and Marge are both puzzled at this.
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 the safe zone. He also tells them that if they see his wife, to tell her that he loves her, but Homer is confused, asking if he should say "I love her" or I meaning him. Doctor Hibbert is then bitten by the Munchers, and Homer says,"Eh, I'll just high-five her." When the family tries to sneak out of the house to take Bart to the safe zone in the car( which is inexplicably unharmed despite most of the town being in ruins) Homer exclaims loudly what a relief it is to feel breeze again after being stuck in the house for weeks, causing all the Munchers in town to swarm the Simpson house. Homer tries to keep the door shut but the sheer number of Munchers causes it to collapse. Homer yells" To the panic room!" as they flee to the front room. Marge responds with" We don't have a panic room!" just as Martin Prince Sr comes down the chimney. They change direction, heading for the living room as Homer yells"To the panic room store!" but are confronted by a zombie Rod and Todd Flanders. Running through the kitchen, Abe Simpson breaks the window and tries to grab Marge but fails.
Rainier Wolfcastle kills the Munchers blocking the front door in an Arnold Schwarzenegger fashion and says "Come with me if you want to live!", but he is immediately bitten by the Munchers. Apu comes just in time to save them in an armoured SUV, running over Hans Moleman in the process. Apu states that being a vegetarian, he didn't eat a burger and as a store clerk, he was prepared for a zombie invasion and thus is stocked with appropriate weapons. As they are going, Helen Lovejoy jumps onto the hood and smashes her hand through the glass. Apu orders Marge to shoot her, but Marge can't, stating that Helen is Lisa's godmother. Apu tells her that she can apologize in Hell, and Marge agrees and shoots Helen off the truck. As they near Springfield's city limits, they travel through the dump. The truck gets stuck at the top of a mountain of junk. Apu gets out to push, stating that Bart must make it to the safe zone. He successfully gets the truck down to the bottom and the Simpsons drive away. Marge talks about how noble it was for Apu to give his life to save them. Apu yells in the distance,"Where are you going?! You were supposed to stop at the bottom and wait for me!" and he is eaten by the Munchers. The car runs out of gas, so they have to walk to the safe zone and protect Bart. Bart uses Lisa as a horse, but Lisa repeatedly walks past a tree branch making Bart hit his head.
While walking, Homer sees Mr. Burns and Smithers lying apparently dead in an overturned car. Homer mocks him saying,"Who's an unproductive worker, now?" Mr. Burns wakes up and bites Homer's hand, turning him into a Muncher. Homer regroups with the family, and Lisa notices that he has turned into a Muncher. Bart pulls the shotgun off Homer and asks him if he wants it in the mouth or eyes. Marge states that they can cure Homer once they get to the safe zone. They lock Homer in a cage and continue walking, and they meet a man who has apparently gone mad. They ask him where the safe zone is and he replies,"Safe zone? You think there's a safe zone?! Well, it's right over there." The family arrives at a post where a soldier says they need to eat Bart to gain his immunity. Marge aims the gun in his face refusing to let them (questioning the logic of someone eating the body and blood of their saviour, with several chistrians sharing nervous glaces)and says there must be another way.
Sometime later, Lisa narrates that the world has returned to normal, people are immune and it would appear that all surviving Munchers were cured. All they had to do to gain the immunity was let Bart bathe in a massive pot of soup they all must eat and everyone is cured—except for Homer, who is still in the cage because he apparently doesn't want to eat food his son has been bathing in.
In the style of a Broadway musical (the entire segment is a stage play, the violence is fake and nobody actually dies), Homer and Marge are at Moe's Tavern together. Homer falls down the door that leads to the boiler room, and 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, everybody in the segment sings "Number XX". To the tune of the Simpsons Theme Song, the cast sings to the audience, saying that they hope they enjoyed this Halloween episode. 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," 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.