"Treehouse of Horror X" is the fourth episode of Season 11, as well as the tenth "Treehouse of Horror" episode. The episode aired on Halloween of 1999, and would be the final "Treehouse of Horror" special to air either before or on Halloween until season 21's "Treehouse of Horror XX" in 2009, a full ten years later.
In "I Know What You Diddily-Iddly-Did" The family runs over and believes that they've killed Ned Flanders, in "Desperately Xeeking Xena" Bart and Lisa become super-heroes and in "Life's a Glitch, Then You Die" Y2K destroys the world and Springfield.
In the opening scene Kang and Kodos open the show in front of an alien audience, in the style of "Rowan and Martin's Laugh-In". No one laughs, and canned laughter is played instead. The Simpsons witness the whole thing on television.
In a parody of I Know What You Did Last Summer, on a foggy evening with a full moon, The Simpsons are driving down a road until Marge accidentally smashes into Ned Flanders, killing him. The next day, Homer climbs to the top of his house with Ned's body, calling down to Maude in her front yard "Hey Maude! Look who's helping me clean the chimney! Maude replies "Oh, I'm so relieved! Whenever you go on one of your late night fog-walks I get so worried" Homer voices Ned, moving him like a puppet "Relax, I'm fine, but when I do die, I don't want any autopsies!". Maude then goes indoors when her pies are finally done. At the same time, Homer throws Ned off the roof, whose corpse lands on the dog house, Homer mutters "Oh, she missed it!". Homer dumps Ned's corpse in his house, waiting to hear Maude's horrified scream. Shortly after Ned's funeral, the Simpsons return home, Homer gloating about how easy it was to get away with murder and attempts to drive over Milhouse ("I never liked that little weiner Milhouse."), with Marge stopping him. The family then discovers that someone knows that
they murdered Ned, writing "I know what you did" on their front door. Late at night during a storm, the Simpsons are sitting on the couch, when the phone rings. Homer answers it and a male voice says "I know you're alone." The voice turns out to be Moe, who was trying to call Maude Flanders, but mistakingly dialed Homer. A shot of lightning fills the their living room with light, showing that the inside of the house is covered in the phrase "I know what you did". After a confrontation with a mysterious cloaked figure wielding a butcher's hook, the Simpsons flee their house. They get into the car but discover that its covered in the same phrase. They escape the figure but the car runs out of gas after Homer prays for God to
save his life. But as the figure closes in, Homer tells them to hide in various places. He tells Marge to hide in an old abandoned amusement park, Lisa to hide in the pet cemetery, Bart to hide in the spooky roller disco, and that he will skinny-dip in the lake where sexy teenagers were killed exactly one hundred years ago. And the figure catches them. They then discover that Ned Flanders was their assailant. Ned was not killed by the accident and he tells them, by an incredibly coincidence, he had been bitten by a werewolf moments before Marge hit him, and werewolves are near impossible to kill. Lisa then puts together that Ned was going to kill them, and they all laugh, but as the clouds drift past the moon, its luminous light engulfs Ned's body and he rips off his clothes as he is now werewolf Flanders. The family flees down the street while Ned mauls Homer to death, with Homer still mocking him "Ooow! Ah. (Flanders struggles to pack Homer into his mouth) eyes bigger than your stomach, eh wolfie? Heh heh... OWWW! Oh God, no!"
In a parody of Desperately Seeking Susan, Marge takes Bart and Lisa to the Springfield Elementary gymnasium, where Chief Wiggum is running a portable X-ray machine to inspect children's candy. Whilst inspecting his son Ralph's candy, he tells them what the candy really contains "safe, safe, razor blade, syringe, oh white chocolate!" But as Nelson's extremely bulky pillowcase of goodies is being scanned, the X-ray machine inevitably malfunctions. Before the explosion, Bart does not duck down to avoid it, and only stares in awe. Lisa stands up to try to get him to get down. The machine bursts out a strong radiation ray, knocking Bart into a gym wall and Lisa into a bleacher, which collapses on top of her, much to Principal Skinner's horror, as Lisa's grades qualify the school for grants. The subsequent exposure to radiation gives Lisa extraordinary strength, allowing her to pick up the book case. Bart gains the ability to stretch any part of his body. They become a superhero duo, calling themselves "Stretch Dude and Clobber Girl".
In their first "adventure", Lucy Lawless addresses fans at a science fiction convention. Comic Book Guy, who has styled himself as a villain called "The Collector", kidnaps her using a magnet to attract her metal breast plate. Lawless goes to remove the breastplate, but when hundreds of fans raise their camera's to photograph her breasts she stops with a statement of "Maybe later." He takes her to his lair, where he puts her in an aluminized Pet film bag for "safekeeping" and imprisons her in a room of other similarly-captured celebrities such as Yasmine Bleeth and Tom Baker. Stretch Dude and Clobber Girl burst into The Collector's hideout, but he manages to knock Clobber Girl out with a working phaser gun that was fired only once to prevent William Shatner from releasing another album. He wastes all his remaining shots on Stretch Dude, who splits in half and does somersaults to avoid the rays. The Collector hurls his empty raygun at Stretch Dude, hitting him on the forehead and knocking him out as well. He suspends the drowsy duo over a vat of bubbling Lucite, slowly lowering them towards certain doom and mocks them "Goodnight Retch Dude & Slobber Girl. Sweet screams!". Lucy Lawless, still in her plastic restraint, lures the Collector over by pretending to be attracted to him. When he's close enough she grabs his lips and then attacks him with Kung Fu moves. Stating that he fell for a "ruse so hackneyed, it would make Stan Lee blush!", the Collector grabs his limited edition double edged lightsaber from Star Wars Episode I: The Phantom Menace and ignites the blades. Lucy reminds him that he has removed it from its original packaging and it is now no longer a collectible. He backs away in horror and plummets into the vat, eventually emerging to strike a dramatic death pose (imitating a Lorne Greene pose from Battlestar Galactica) before the plastic hardens around him. Lawless flies Stretch Dude and Clobber Girl home to safety on her back. Clobber Girl is confused, and points out that Xena can't fly. Lucy responds that she isn't Xena; she's Lucy Lawless. This explanation satisfies Clobber Girl and the trio fly away to a reprise of the 'Stretch Dude and Clobber Girl' theme tune.
"Good-bye, Lisa. Remember me as I am-- filled with murderous rage!"
--Homer, as Lisa and Marge leave for Operation Exodus.
On December 31, 1999, Dick Clark celebrates his New Year's Rockin' Eve in Springfield instead of Times Square. Homer, the Springfield Nuclear Power Plant's Y2K compliance officer, confidently declares that he fixed every computer at the power plant. Unfortunately, Homer didn't fix his own computer, which creates the computer virus that spreads rapidly to other systems all over the world. But as Dick Clark's New Year's ball drops and hits "1999", the electronic year display reads "1900". Within minutes, Dick melts to death (he is shown to be a robot) and chaos breaks out as airplanes crash, appliances explode, malfunction and turn evil, including the Springfield Revolving Restaurant's Sit 'n' Rotate room dislodging from its stand and flying off, presumably into space. A widespread looting soon begins and as the family roams the streets observing the damage (including traffic lights that shoot multi-colored beam blasts and mailboxes walking by themselves), Krusty's pacemaker sets itself to hummingbird speed and he collapses in front of them. A letter in Krusty's pocket states that a rocket is being populated with humanity's "best & brightest" and will be launched shortly in an effort to preserve human civilization on Mars. When they reach the rocket, Homer unsuccessfully attempts to bluff his way on board (claiming to be the piano genius from the movie Shine), but the armed guard recognizes Lisa as having a seat reserved on the craft, knowing that she's the ship's proofreader. Lisa is only able to take one parent with her, and she quickly chooses Marge, who then takes Maggie along. Homer and Bart soon find a second, unguarded rocket nearby and climb on board just before it launches. However, they quickly notice that this ship is filled with Ross Perot, Dan Quayle, Tonya Harding, Albert Sharpton, Courtney Love, Spike Lee, Tom Arnold, Pauly Shore, Rosie O'Donnell and Dr. Laura and the like: it is deliberately set for a collision course with the sun. Unable to bear the short trip to oblivion with the B-list celebrities, Homer and Bart eject themselves into the vacuum of space, where they sigh in relief as their heads swell up onscreen and explode offscreen while the rocket heads towards the sun, where the passengers sing The Trolley Song before they die of explosive decompression. This is a parody of When Worlds Collide.