"Treehouse of Horror XVIII" is a Halloween-themed episode from the nineteenth season of the animated comedy series The Simpsons. It was first broadcast in the United States on the Fox Network on November 4, 2006.
In common with all the previous "Treehouse of Horror" episodes, the show is made up of three self-contained stories; "E.T. Go Home", "Mr. & Mrs. Simpson" and "Heck House". The first story parodies the 1982 movie E.T. the Extra-Terrestrial. In the second story, which parodies the 2005 movie Mr. & Mrs. Smith, Marge and Homer Simpson discover that they are both secret assassins and have been hiding the fact from each other. In the final story, Ned Flanders, the Simpsons' pious Christian neighbor, asks God to give him the power to frighten the children of Springfield into changing their ways. He is transformed into the Devil (a role which Flanders had previously played in "Treehouse of Horror IV") and takes the children to Hell.
Marge Simpson is seen preparing cupcakes with jack-o-lantern faces on them, although she reminds viewers that Halloween was the previous week. As she speaks, icons advertising other programs on the Fox Network appear at the bottom of the screen and miniature people, representing characters from those shows, emerge from the icons. Marge attacks the miniature people with a knife, cutting them up and mixing them into the meatloaf which she is making. Marge serves the meatloaf to her husband Homer, who finds it delicious. As the meatloaf is cut into slices, the viewer sees the body parts of the miniature people inside. The body parts form the words "Treehouse of Horror XVIII".
"E.T. Go Home"
When Bart Simpson is told to fetch some butane gas from the shed, he finds the alien Kodos hiding there. Although it is hinted at that Kodos has been sent to Earth to destroy mankind, Bart takes him at his word that he came in peace. Bart takes the alien into his house. He and his sister Lisa help Kodos to gather the components which he says he needs to contact his home planet. When their parents Homer and Marge discover the alien, they allow him to stay, Kodos having accused them of being prejudiced and only wanting him out of the house because he is Jewish, and allow the children to continue helping him.
It is eventually revealed that the parts which Kodos had been collecting were to build a portal to connect Earth with his home planet. After the portal has been constructed, an alien invasion of Earth begins. However, following a brief war, the aliens are defeated.
The Simpson family are invited to witness Kodos' dissection. Kodos points out that it is technically a vivsection because he is still alive. Homer Simpson responds to that by smothering the alien with a pillow.
"Mr. & Mrs. Simpson"
Homer and Marge Simpson are seen apparently attending marriage guidance counselling, discussing recent difficulties that they have had in their relationship.
A flashback reveals that Homer and Marge have both been leading a double life as highly skilled assassins. Homer was sent to assassinate the TV anchorman Kent Brockman but was beaten to the kill by Marge. Homer's jealousy becomes a murderous rage. He and Marge try to kill each other, murdering many bystanders in the process. When the two of them together cause the death of chief of police Chief Wiggum, they find out that killing people together reignites their love for each other.
The segment ends by revealing that Homer and Marge are not at marriage guidance counselling, they are at Springfield Elementary School in the office of Principal Skinner. The principal, who had asked them there to discuss the behavior of their son Bart, does not understand why they are telling him about their relationship problems. The couple decide to ignore Bart's behavior problems and kill Skinner for fun.
On Halloween night, Bart Simpson in a Frankenstein's monster costume, Lisa Simpson in a witch costume and other children are out trick-or-treating. They are disappointed that some of their neighbors do not give them much candy and decide to play pranks on those neighbors who have not given them many treats. The pranks soon develop into more serious vandalism. leading the people of the neighborhood to conclude that the children need to be taught a lesson.
Ned Flanders, the pious Christian who lives nextdoor to the Simpsons, offers to help. He sets up a "Heck House" (a kind of haunted house attraction) at the church. His intention is to scare the children into changing their behavior by showing them how they will be punished in Hell for their misdeeds. The curious children decide to have a look at the "Heck House" but do not find it scary at all, it mostly consists of Flanders' sons Rod and Todd doing unconvincing role plays.
Flanders prays to be given the power to scare the children straight. He is transformed into the Devil, takes the children to Hell and shows them how all of the Seven Deadly Sins are making the lives of the people of Springfield miserable. The terrified children agree to behave themselves in future.
The segment ends with Flanders telling the audience that they will be going to Hell for watching the Fox Network.