"Treehouse of Horror X" is a Halloween-themed episode from the eleventh season of the animated comedy series The Simpsons. It originally aired in the United States on the Fox Network on October 31, 1999. It was the last of the Simpsons Halloween specials to have its first broadcast either on or before the date of Halloween until "Treehouse of Horror XX" in 2009. All of the intervening Simpsons Halloween episodes originally aired in early November.
In commmon with all the previous "Treehouse of Horror" episodes, the program is made up of three self-contained stories; "I Know What You Diddly-Iddly Did", "Desperately Xeeking Xena", and "Life's a Glitch, Then You Die". The first story is a parody of the 1997 movie I Know What You Did Last Summer. In the second story, Bart and Lisa Simpson become superheroes. The final story makes light of the Y2K bug, a serious source of concern at the time that the program was first shown.
The aliens Kang and Kodos are presenting the Halloween episode of a television variety show. They emerge from behind a facade shaped like a jack-o-lantern and begin telling bad jokes. The studio audience of other space aliens are clearly unamused but raucous canned laughter can be heard.
The Simpson family are sitting on their living room couch and watching the aliens' TV show. Except for Lisa, all of the characters take the unusual forms in which they had been seen in previous Simpsons Halloween specials. Homer is a human jack-in-the-box, as in "Treehouse of Horror II". Marge is a witch and Bart is a fly-headed monster, both forms which had previously been seen in "Treehouse of Horror VIII". The baby Maggie is a human-alien hybrid, as in "Treehouse of Horror IX".
Lisa, unimpressed with the TV show she and her family are watching, comments that aliens have nothing to do with Halloween. Maggie orders her to be silent, before zapping her with a ray gun.
"I Know What You Diddly-Iddly Did"
Thanks to the strength that they gained from eating some sugary breakfast cereal, the Simpsons manage to escape from some vampires. As they drive home on a foggy evening, they accidentally hit and kill their neighbor Ned Flanders. To cover up their crime, Homer takes Ned's body home with him. The following morning, Homer is sitting on the roof of his house with Ned's corpse. He uses the body as a puppet and calls out to Ned's wife Maude, telling her (as Ned) that he is fine now but does not want an autopsy after he dies. Homer wants to fool Maude into thinking that Ned has accidentally fallen off the roof. However, Maude is not looking when Ned falls. Homer dumps the body in his neighbors' home instead.
After Ned's funeral, the Simpsons find the words "I Know what you did" written all over their house and car in blood. A cloaked figure brandishing a butcher's hook chases them out of their house and pursues them into the wilderness, where their car runs out of gas. The cloaked figure reveals himself to be Ned Flanders. He explains that moments before he was struck by the Simpsons' car, he was bitten by a werewolf. He became a werewolf too, consequently becoming almost impossible to kill.
Clouds part, revealing the full moon, and Flanders transforms into a werewolf. The other members of the Simpson family escape. The werewolf Ned begins to attack Homer, although Homer mocks him as he does so.
"Desperately Xeeking Xena"
On Halloween night, the costumed children line up at Springfield Elementary School where an x-ray machine has been set up to scan the candy which they got from trick-or-treating. When the machine malfunctions, Bart and Lisa Simpson gain superpowers. Bart acquires the ability to stretch his body like rubber and Lisa gains superstrength. They form a superhero duo, calling themselves Stretch Dude and Clobber Girl.
The owner of Springfield's comic book store sets himself up as a supervillain, calling himself The Collector. He abducts Lucy Lawless, the star of Xena: Warrior Princess. He places her in a gigantic polythene bag and puts her with the other identically stored fantasy and science-fiction stars in his collection. Lisa and Bart break into The Collector's lair but he captures them too and suspends them over a vat of hot plastic. Lucy Lawless pretends to be romantically interested in The Collector. She gets his attention and then attacks him. The Collector announces that he will defend himself with a Star Wars: Episode I The Phantom Menace light sabre. Lawless points out that The Collector has taken his prized light sabre out of its original packaging, thus making it less valuable. The Collector panics and falls into the vat of plastic.
Lawless rescues the children and flies them home. Lisa points out that Xena is not able to fly. The actress reminds her that she is not Xena but Lucy Lawless.
"Life's a Glitch, Then You Die"
It is December 31, 1999. Homer Simpson is the Y2K compliance officer at Springfield's nuclear power plant. He proudly announces that he has made sure that everything at the plant is Y2K compliant, however, he has neglected his home computer. Homer's computer sends a virus across the world, causing appliances to malfunction, become evil and put people's lives at risk.
As a result of the Y2K bug, Krusty the Clown's pacemaker stops working and he collapses and dies in front of the Simpsons. The Simpsons find a letter in Krusty's pocket, which says that the "best and brightest" of Earth's population have been selected to leave on a rocket and begin civilization anew on Mars. The Simpsons head towards the rocket. Homer claims to be the musical genius whose life story was told in the movie Shine, saying that his name is Shiny Mc.Schine. The rocket's guards see through his feeble ruse and deny him access. Lisa Simpson, however, has already been selected to leave on the rocket. She is allowed to take one parent with her and chooses to take her mother Marge.
Homer and his son Bart notice a second rocket which is unguarded. They go on board but soon notice that the passengers are not the Earth's "best and brightest" but annoying B-list celebrities. They find out that instead of going to Mars, the second rocket is headed straight for the sun. Knowing that they are going to die anyway, Homer and Bart choose to leave the rocket and go out into space, where their heads explode, rather than spend any more time than they have to with the irritating celebs.