Treehouse of Horror XXVII is the 27th Halloween episode of The Simpsons, and also notably the show's 600th episode.
Homer takes the family to buy a Christmas tree on Halloween night with the excuse that "in America, everything's way too early". At the christmas trees lot, the family is surprised by Sideshow Bob, Kang or Kodos and the ghost of Frank Grimes, who want to get their revenge on the Simpson family. However, a Leprechaun appears and convinces the trio to dance. Maggie grabs a knife and cuts their heads off.
After the title screen, Frank puts his head back on and questions about "stopping these people". The screen is then filled with clips from all the episodes so far and a counter goes from 0 to 600. ("In Hell, they make you watch them all in a row!")
The segment takes place on a post-apocalyptic Springfield, where the city has completely gone dry, and the very little water there still are is very expensive. Mr. Burns, who controls all the water in the town makes an announcement to them, saying that a child from each neighborhood has to participate on a battle to death, and who wins will be awarded a day at Burns' reservoir. Lisa is the selected child from the Nevergreen Terrace neighborhood, and she soon met her coach, Homish. At the battle, Ralph dies before the event starts, as he was blown when he stepped out of the area.
Later, two boys manage to seduce Lisa, but Homish kills them. Later, Homish falls on a trap and is surrounded by other contestants, but Lisa convinces them that they don't have to kill each other. She also convinces the population of Springfied to turn against Mr. Burns. They free all the kids from the dome and destroy Burns' reservoir, which they immediately regret as the water was completely wasted. However, the water that escaped the reservoir caused a rain, that caused a flood and later an ice age.
Lisa is playing hide and seek with Janey when a lawnmower turns on and kills Janey. At her funeral, Sherri and Terri say that as Lisa has lost her friend on a tragedy and is now interesting, she can be their friend, but they are immediately crushed by a falling tombstone. To forget the trauma of losing three friends in two days, Lisa goes to a psychiatrist, who says to Lisa that she can think of her as her best friend, but the doctor is killed by a falling painting.
At the doctor's funeral, Lisa starts being investigated for the murderer of her friends. The police finds sparkle nail polish on Lisa's room and claims that it was found on all four crime scenes. Lisa reveals that she only used it with her former imaginary friend Rachel, but she eventually outgrew her.
The next day at the school bus, all the kids fear Lisa except Milhouse, but Rachel appears and asphyxiates him with a plastic bag. Chief Wiggum sees Lisa on the bus and arrests her. At jail, Lisa mentions that Marge was right about Rachel, which infuriates her even more. Lisa soon realizes that Rachel will try to kill Marge and escapes jail with Bart's help.
Lisa gets home in time to save Marge from being killed and Rachel reveals her plan to kill everyone who Lisa loves and people will blame Lisa as nobody can see her. Rachel tries to kill Homer, but he occupies her with his imaginary friend: Sargent Sausage. Lisa then realizes that as Rachel only exists in her imagination, she can turn her into anything she wants, so she turns Rachel into her own mother, who is married to a dentist.
Bart is being chased by Dolph, Jimbo and Kearney when he enters Moe's Tavern through a backdoor. Moe appears, incapacitates the bullies and shows a secret underground area on the tavern, where all barflies are secret agents saving the world. He then reveals that he wants Bart to take Homer's place, one of their best men who was lost in action.
Later, they receive information on their next target: Remoh Industries, a multi-billionare company and the new owner of Duff Stadium. At the company, they break in and find out that the company's boss is Homer. He then reveals that he has created a lava machine to make all the world surrender to him and he wanted all the beer to celebrate. When the agents were going to attack Homer, he sends a horde of people to fight them Bart manages to escape the fight alive and kills his own father.
- Homer appeared on Jimmy Kimmel Live! to promote the episode.
- Hans Moleman's car plate is "VABF16", the production code of the episode.
- Russ Cargill from The Simpsons Movie appeared in promos for this episode as one of the villains trying to kill the Simpsons in the opening segment, but in the actual episode, he was replaced by the Leprechaun from "Treehouse of Horror XII".
- This is the first time Kang or Kodos doesn't speak Rigellian (which, by coincidence, is identical to English).
- Judith Owen, in addition to voicing Mrs. Mancuso-Gluckman, also sang the 600 theme at the end of the episode.
- Donald Fagen's "I.G.Y. (What a Beautiful World)" was previously heard in "Future-Drama".
- Moe references the fact that the segment is part of the Halloween special.
|Dan Castellaneta|| Homer Simpson|
Krusty the Clown
|Julie Kavner||Marge Simpson|
|Nancy Cartwright|| Bart Simpson|
|Yeardley Smith||Lisa Simpson|
|Hank Azaria|| Frank Grimes|
|Harry Shearer|| Mr. Burns|
|Kelsey Grammer||Sideshow Bob|
|Judith Owen||Mrs. Mancuso-Gluckman|
|Pamela Hayden|| Milhouse Van Houten|
|Tress MacNeille||Dolph Starbeam|
|Russi Taylor|| Sherri|
|The Simpsons Treehouse of Horror|
See also: Halloween of Horror