"Treehouse of Horror XXIX" is a Halloween episode of the long-running American animated comedy series The Simpsons. It originally aired on the Fox network in the United States on October 21, 2018. In common with most previous Halloween episodes of The Simpsons, it is made up of several short, self-contained stories that all feature recurring characters from The Simpsons but which are not considered to be a part of the series' normal continuity by the show's creators or its fans.
The episode's pre-credits opening sequence sees Homer Simpson defeat Chthulhu, the monstrous octopus-like god from the writings of H.P. Lovecraft. The segment "Intrusion of the Pod-y Snatchers" is a parody of the 1956 science fiction horror film Invasion of the Body Snatchers. The segment "Multiplisa-ty" is a parody of the 2016 psychological horror film Split. The segment "Geriatric Park' is a parody of the 1993 science fiction adventure film Jurassic Park.
The Simpson family, made up of Homer Simpson, his wife Marge, the ten-year-old boy Bart, the eight-year-old girl Lisa and the baby Maggie, visit a strange and gloomy seaside town where Homer hopes to enter a clam eating contest. Homer is told that he will be sacrificed to the great god Chthulhu. Before Chthulhu eats him, Homer demands to take part in the clam eating contest that he was promised. Chthulhu agrees and competes against Homer. Being a great glutton, Homer is easily able to eat more clams than Chthulhu. The defeated Chthulhu says that he will grant Homer any request. Homer says that he wants to eat Chthulhu.
"Intrusion of the Pod-y Switchers"
It is revealed that the company that produces the most popular smartphone is controlled by aliens that are intelligent humanoid plants. The aliens plan to use the smartphones to keep people distracted so that they will not notice that Earth is being invaded. The people of the Simpsons' town of Springfield are consumed by alien pants. The plants then produce copies of their victims. Bart and Lisa Simpson watch as almost all of the people of Springfield are replaced by zombie-like plant people. Eventually, they too succumb to the alien plants.
After being consumed by an alien plant, Lisa finds herself on the aliens' distant planet. The rest of her family and all of the other people of Springfield are all already happily living on the paradise-like planet. One of the aliens explains that they brought the Earthlings to their planet to save them from themselves and allow them to live a happy and simple life without the constant distraction of smartphones. The alien then sees that all of the people of Springfield are looking at smartphones again. He asks them where they got the phones. Bart replies that they found them wrapped up under another alien, a living Christmas tree.
Bart Simpson is joined by his friends Milhouse Van Houten and Nelson Muntz for a sleepover. The three boys drink some drugged orange drink and pass out. When they come to, they find themselves locked in a cell. Bart's sister Lisa opens the cell door. She is calling herself Penelope and acting strangely. It becomes clear that she now has multiple personalities.
Some time later, Lisa tells the boys that she is going to recycle them. Nelson is thrown into an incinerator. Milhouse is thrown into a machine that processes paper for recycling. Bart is about to be crushed by a huge pile of trash. He asks Lisa what has happened to her.
A flashback reveals that Bart took one of Lisa's spelling tests, altered the answers to make them all incorrect and added insults directed at Lisa's teacher. As a result, Lisa got a failing grade, causing her to snap and lose her mind.
Lisa tells Bart that she will give him one last chance to say something to redeem himself. Bart makes a speech in which he tells Lisa that he will always be a loving and supportive brother to her. Lisa is moved by what he says. Her normal personality is restored and Bart is saved. It is revealed that Milhouse is also still alive, although he is now part human and part paper.
Mr. Burns (who is Homer Simpson's boss at the nuclear power plant in the show's regular continuity) opens a retirement home on an uncharted island where old people are rejuvenated by being injected with dinosaur DNA. Most of the old people of Springfield move to the island, including Marge Simpson's mother Jacqueline Bouvier and Homer Simpson's father Abraham Simpson. The dinosaur DNA makes them all much stronger and more active than they were before and makes them happier as a result.
Homer, Marge, Bart and Lisa go visit the island. When Abraham Simpson mentions that he is feeling a little cold, homer turns up the thermostat, paying no attention to a sign warning him not to do so. It is revealed that the cool temperature was the only thing preventing all of the old people from turning into dinosaurs. When the temperature rises, they all transform and go on the rampage, attacking and eating visitors to the island.
Lisa Simpson thinks that she can prevent her grandfather from attacking her and the rest of her family by appealing to his humanity. She tells him that she understands that he just wants to be seen as a person and not as a strange scary thing called a senior citizen. She adds that she wants to hear her grandpa's opinion about everything. Lisa's plan works perfectly. Although they still look like dinosaurs, all of the residents of the retirement home start behaving like regular old people again.
Mr. Burns begins to gloat, saying that he was the true monster of the story but he escaped unpunished. He is then eaten by the Old Jewish Man dinosaur.