Treehouse of Horror XVI was the fourth episode of the seventeenth season of The Simpsons, as well as the sixteenth Halloween episode. The episode aired six days after Halloween. This is one of several Simpsons episodes which are considered anthology episodes that features mini-stories.
In "B.I. Bartificial Intelligence" Bart lapses into a coma, Homer and Marge replace him with a robot boy. Homer competes in "The World Series of Manhunter" with Mr. Burns hunting him, and Terry Bradshaw as the host in "Survival of the Fattest". A witch casts a spell at a Halloween costume party, turning everyone into what their costume is in "I've Grown a Costume on Your Face".
In the opening, using an accelerator-beam, Kang and Kodos hope to speed up a World Series baseball game to air The Simpsons. Things later go sour, and the baseball stadium is drawn in to a specific point where the gravity is high. Afterwards the stadiums' surroundings are drawn into this point and then the whole city of Springfield, then Earth's oceans, the whole Earth, the solar system, numerous galaxies and finally God is drawn into this singularity, a form of a Big Crunch. Kodos says to Kang, "Smooth move, space-lax! You've destroyed the totality of existence." Kang replies, "It will be fine. I'll just leave a note." The Yellow Post-it note he places says: "Treehouse of Horror XVI" in red writing that looks like blood.
In a parody of A.I. Artificial Intelligence, Bart ends up in a coma after attempting to jump from the roof to the swimming pool at Spinster Arms Apartments. In an effort to cope with the loss of their boy, the family takes in a robotic boy, named David, who quickly proves to be a better son. Later, Bart wakes up from the coma however the two boys soon try to compete for their families love. Marge grows concerned with the boys rivalry, Homer comforts her and says he'll fix it. Of course instead of getting rid of the better robot son Homer decides to dump Bart in the woods. Bart soon comes across a band of defunct robots, harvesting their parts to become a powerful cyborg. He returns home to seek his revenge on David, and in the end, he cuts both Homer and David in half. Eventually, Homer, who no longer has his legs, now has to be fused with David's legs. In the end it is revealed that it was all a dream induced by Homer's Demonic Possession, in a parody of 'The Exorcist'. A priest comes and splashes holy water on Homer who begins to preform the exorcist movements and music, and climbs up on the ceiling. Marge calls the plant to tell them that Homer won't be able to come in today, and Homer exclaims "Whoo-hoo!" and collapses on the bed, relaxed, but with his head still turned around backwards.
In a parody of The Most Dangerous Game, Homer and other citizens of Springfield receive a letter from Mr. Burns to come to his mansion to go hunting. Homer goes, despite his family's warnings. Unbeknownst to them, they are the prey to be hunted by Burns. After his lawyer writes something that "should work in just about every courtroom" (and is then shot by Burns), he gives everyone five minutes vantage, but even so he manages to kill almost everyone, some of who climbed a tree but were launched into the air by Homer, and shot by Burns (except for Moe, who dies because he's impaled on a weathervane and being crushed by a dead Chief Wiggum).
While the Simpson family watch the hunt on TV, the survivors hide in a forest, in which Professor Frink is eaten by Homer and everyone but him is shot by Burns' attack helicopter.
Homer manages to survive the night as his friends are killed, and he uses Barney's dead body so Burns thinks he is Homer. He falls from a tree and Burns is ready to shoot at him, but just as he is about to do it, he and Smithers are both knocked out with frying pans by Marge, who then hits Homer for being away from home for eighteen hours and not calling home once. Homer can't stay mad at Marge as the pan she hit him with smells like sausages and onions. Marge replies by admitting that she's glad that Homer's okay.
Homer and Marge kiss each other at first, but then go as far as deciding to make out with each other. One reporter asks Terry Bradshaw what his thoughts are on what's just transpired. Terry Bradshaw considers this a disgrace, as the network will show a dozen gruesome murders, but bets that they'll cut to commercial before Homer and Marge begin to have sex, describing it as "the tender act of love." He is interrupted by Homer unzipping Marge's dress while making out with her, with Terry noting that he's spoken too soon, turning around quickly, cheering at both performing sex, excitedly saying "There they go", blocking the viewer from what's shown. It ends with him shockingly saying that what he's seeing is going in his Playbook, as he watches both continue to have sex on national television.
In a parody of Halloweentown II: Kalabar's Revenge, the citizens of Springfield dress in their Halloween costumes for a costume contest. A wicked witch also entered, and won. But after revealing that she isn't wearing a costume, and is really a witch, she is disqualified and her prize is taken away. In revenge, she turns everyone into the actual characters that their costumes are based on. All of the citizens have problems with their appearances, including the Simpsons: Homer's head falls off, Marge turns into a skeleton (but with her own Human head), Bart into a werewolf, Lisa into Albert Einstein and Maggie into a witch (as they were all dressed like that). Lisa tries to discover how to reverse the spell, then she realizes Maggie is now a witch and she can do it. The citizens come to their house, some saying they like their appearance and abilities, and some saying they want to be back to normal. Notably, there is Flanders as a flower, being followed around by a bee (Bumblebee Man), Apu as the droid R2D2 from Star Wars, nicknamed ApuD2 and Milhouse as a gold bulldozer. Maggie gets confused, so she eventually turns the people of Springfield into pacifiers with their normal heads and goes away on her broomstick. Where Moe then tells the audience that he hopes they have enjoyed watching this years Halloween episode as much as the Koreans did doing the animation. Along with a brief public service announcement on adult illiteracy Meanwhile Dennis Rodman (who is also as a pacifier) comes and says he is working off a speeding ticket. Maggie comes back and casts another spell and the credits appear, presumably changing everyone back to normal.