The Simpsons return in their twentieth "Treehouse of Horror" special with three tales of diverse terror.

''Dial M for Murder or Press # To Return to the Main Menu'': In a send-up of Alfred Hitchcock's classic suspense films (with ''Strangers on a Train'' being the main reference), Bart gets Lisa involved in a "criss-cross" revenge scheme, but while Lisa thinks the plan is just harmless practical jokes, Bart sees it as an opportunity for murder.

''Don't Have a Cow, Mankind'', In this bizarre ''Film/TwentyEightDaysLater'' meets ''Film/ChildrenOfMen'' tale, Springfield is once again overrun by zombies ([[NotUsingTheZWord now called "munchers"]]), this time, thanks to Krusty's latest fast food sandwich made from cattle with mad-cow disease -- and the only one not affected by the tainted meat is Bart.

''There's No Business Like Moe Business'': In this parody of Broadway musicals (particularly ''Sweeney Todd''), Moe buys a microbrew machine to make a new beer -- and Homer's blood from getting impaled on the machine turns the microbrew into Moe's most popular drink.

This was the first "Treehouse of Horror" episode since season 11's "Treehouse of Horror X" to air around Halloween due to Fox's World Series broadcasting rights.

!!This episode contains examples of: ([[YMMV/TheSimpsonsS21E4TreehouseOfHorrorXX YMMV Examples here]])
* BadassNormal: Dr. Hibbert during "Don't Have a Cow, Mankind" when he manages to fend off several Munchers while telling the Simpsons to get Bart to the safe zone over the phone. Credit goes to Apu as well, for coming to the Simpsons' aid with an armored truck and assortment of guns (which he has to have, as he's a convenience store clerk and would need them in case of a robbery). Sadly, the Munchers get both of them.
* BigDamnHeroes: Parodied as Rainer Wolfcastle comes in, gunning down Munchers and tells the family to [[Film/TheTerminator come with him if they want to live]]. He promptly gets overwhelmed and eaten, leading Homer to quip [[TakeThat "Another politician who can't keep his promises"]]. Apu plays this more or less straight.
* BodyHorror: Homer impaled on the pipes of Moe's beer distiller and managing to stay alive despite it
* CampGay: One of the song numbers in "Moe Business" portrays Homer as this during Moe's attempt at convincing Marge that Homer left her because he came out of the closet.
* ComicallyMissingThePoint: When Kent Brockman becomes the first Muncher, he takes a big bite out of Krusty's arm. The clown angrily shouts "Hey! That's my check-cashing arm, you stupid-!" before becoming a Muncher himself.
** Later, when Homer becomes a Muncher, Bart then pulls a shotgun to kill him, asking if he'd want it in the mouth or eyes. Homer giddily requests the mouth.
* CrazyPrepared: Apu is the most prepared for a zombie apocalypse because, as a Hindu and a vegan, he's one of the few people who isn't infected by the Krusty burger (since both vegans and Hindus aren't allowed to eat meat or meat byproducts), and, as a convenience store clerk, he has to have a wide assortment of guns and weapons in case of robberies.
* DeliberatelyMonochrome: The first segment is styled as such, befitting its parody of Hitchcock films.
* DepartmentOfRedundancyDepartment: Moe's lyric "I'm unlucky in love./I'm unlucky in luck."
* ForHalloweenIAmGoingAsMyself: The Franchise/UniversalHorror monsters attempt this, but when their "costumes" are dismissed as lame by the bully kids, they head to the costume shop to buy more current costumes.
* GettingCrapPastTheRadar: "You'll be sucking your own blood tonight!", as said by Dracula's angry wife.
* HeroicSacrifice: Subverted. When the trunk gets stuck on the top of a mound of garbage, Apu volunteers to get out and push the car unstuck with the munchers close in pursuit. He succeeds, but the Simpsons drive off without him, believing he was sacrificing himself to save them when he actually wanted them to park at the bottom of the mound and wait for him.
* HulkSpeak: Frankenstein's Monster and Nelson's mother share an exchange of "Good", "Beer Good", and "You Good" in the opening.
* ImAHumanitarian: The special ingredient to Moe's brew being Homer's blood. [[LampshadeHanging Lampshaded]] in the lyrics
--> '''Lenny''': Full-bodied!
--> '''Carl''': Full-blooded!
--> '''Barney''': It's such a lovely blend!
--> '''All Three''': It's jolly! It's loyal! Like drinking your best friend!
* ImpaledWithExtremePrejudice: In his idiocy, Homer goes for the unattended tap in "Moe Business" and falls onto the exposed piping of the beer distiller.
* [[MusicalEpisode Musical Segment]]: "There's No Business Like Moe Business"
* MythologyGag: The title of the second segment, "Don't Have a Cow, Mankind", plays off one of Bart's earlier catchphrases.
* NoFourthWall: "There's No Business Like Moe Business" due to the segment's nature as a play
* NotSoDifferent: During their final confrontation on the merry-go-round at the playground, Bart argues that Lisa is no different than him. She argues otherwise, throwing the knife into the air. Due to the merry-go-round's rotation, it lands and kills Bart anyway. And while Lisa initially states it was an accident, her interaction with Mrs. Krapabbel at the end implies Bart was correct.
* NotUsingTheZWord: Parodied in "Don't Have a Cow, Mankind" where the zombies are referred to as "Munchers" due to their cannibalistic tendencies and that they became zombies due to munching Krusty's newest burger.
* OffWithHisHead: After Homer attempts to sway the anger of the Monsters' angry wives in the ColdOpening, he's promptly attacked and beheaded. His head lands in the bowl of punch with WingdingEyes providing the "XX" of the episode title.
** Bart seems fond of decapitation in the first segment, judging by how he shows Lisa Ms. Hoover and Principal Skinner's heads (the latter appearing in the promotional art). He also attempts to pressure Lisa in decapitating Mrs. Krabappel with the blade of a paper guillotine. She doesn't go through with it.
* RealityEnsues: In "Moe Business", after the aforementioned musical number about Homer being gay, Moe heads to check on Homer in the distiller...only to be reminded that he had to get ''out'' of that costume and make-up in order to do that song. Moe is left awkwardly stalling to the audience while Homer gets back into costume offstage.
* RoaringRampageOfRevenge: Lisa tries to kill Bart to stop his torment and murders. She succeeds.
** Homer pulls this at the end of "Moe Business", managing to leap into the scene despite still being attached to the distiller, and "killing Moe" (more accurately, throwing a cheap ragdoll of Moe aside).
* TheSecretOfLongPorkPies: Moe's beer becomes more popular after it gets mixed with Homer's blood.
* SheetOfGlass: Played straight, apparently Munchers need windows too.
* ShoutOut:
** In the ColdOpening, Dracula organizes a meeting with the Werewolf, Frankenstein's Monster, and the Mummy on Halloween night filled with Simpsons characters dressed up in notable costumes. After being called "lame-ohs" by the local bullies, they don the costumes of Film/IronMan, Franchise/HarryPotter, WesternAnimation/SpongeBobSquarePants, and [[Franchise/PiratesOfTheCaribbean Jack Sparrow]].
** During "Moe Business", Barney is seen wearing a [[Theatre/ThePhantomOfTheOpera Phantom]] costume.
** While Lisa chases Bart during the climax of "Dial M for Murder", they pass through a montage of classic Hitchcock films.
* SoundtrackDissonance: The credits, similar to ''Treehouse of Horror VI'', although [[JustifiedTrope justified]] due to it being an instrumental version of the theme of "Moe Business", the final segment.
* SpecialEffectsFailure: In-universe. Homer "killing" Moe in "Moe Business" is clearly just him beating up a dummy that looks like Moe. And he doesn't put a lot of effort into it either.
* StrangersOnATrainPlotMurder: Bart's "criss-cross" setup in "Dial M for Murder".
* TakeThat: In the second segment, as Homer tries to bludgeon Bart in his bedroom with the last book on Earth for becoming a Muncher, Lisa bursts into the room and begs him not to do it...only to discover that said book is an autobiography of Arsenio Hall, which causes her to relent.
* TooDumbToLive: Homer as usual. During "Don't Have a Cow, Mankind", he tells Bart to remain quiet when they attempt to escape. Unfortunately, he then yells "Man, that breeze feels good!", drawing the attention of the Munchers in range.
** Bart in the same segment, is starving (he refuses to eat the large amount of fresh fruit and vegetables they have) so he goes outside when he spots an uneaten Krusty Burger, one he knows full well is made of meat that turns people into zombies. The boy risked his life and his families lives because he would rather eat a month old toxic burger than fresh fruit. Fortunately he just so happens to be immune.
* WingdingEyes: Homer's head is ripped and his pupils become the two 'X's from "Treehouse of Horror XX".
* YouKeepUsingThatWord: Parodied with Kang and Kodos' cameo in the audience of "Moe Business":
--> '''Kang''': This is the best musical in lightyears!
--> '''Kodos''': Lightyears measure distance, not time.
--> '''Kang''': You know what I meant!
* YouMonster: Lisa declares Bart is a monster after seeing he's murdered Hoover.
* ZombieApocalypse: A "Muncher" Apocalypse, caused by Krusty's newest burger no less. Eventually reversed when the safe zone has Bart, who is resistant to the virus, bathe in their food supply, presumably after killing all Munchers save a still-infected Homer.