Opening sequence

The episode starts in Fogburyport, birthplace of Green Clam Chowder. The Simpson family arrive there because a book stated it's a place to visit before they mysteriously disappear.

However, it was revealed to be a trap and that they will be devoured by Cthulhu. However, Homer states that he was promised an oyster eating contest against Cthulhu, and Homer beats him. Cthulhu pukes and asks what Homer wants as a reward. Homer whispers "I want to eat you."

Cthulhu is seen in a giant cooking pot and the Simpsons (except Lisa) enjoy a hot dog made of Cthulhu's tentacles.

Homer then pokes Cthulhu's head with a trident, and his ink spells the title of the episode and the opening credits.

This sequence is a reference to the novel The Shadow over Innsmouth by H. P. Lovecraft.

Intrusion of the Pod-y Switchers

The segment begins at the underwater base of Mapple, where Steve Mobbs (on a screen) tells about the new Myphone and the new unfunny version of himself to an excited audience. When everyone is on their phone, the unfunny Steve is revealed to be a plant alien on a mission. Now on the plant planet, they shoot spores to Earth (going right next to the Futurama ship with a cloth saying "BRING BACK FUTURAMA" and then getting blown up by The Orville). In Springfield, everyone turns into plant versions of themselves. The transformed citizens are transported to a utopian plant planet with no technology. The Plant People see the citizens with pods (Mapple products) and ask where they found those. Bart says they found it under the living Christmas tree.

Multiplisa-ty

After a sleepover at Milhouse's, he, Nelson, and Bart find themselves in a cell, trapped by Lisa, suffering from dissociative identity disorder (similar to James McAvoy's character from Split), who closes them in again after they do not ask for an encore on her performance.

During a series of personality changes, Lisa attacks and kills Nelson, and transforms Milhouse into a "paper boy" before telling Bart what he did to anger her: Bart grabbed Lisa's spelling test, changing her answers, mocking Miss Hoover and granting her a F. Lisa has a last change and gives Bart a last chance to save himself and he apologizes and pledges his brotherly love for her, saving himself from being killed by trash. Returning to her normal self, Lisa confesses to Marge, admitting that the lack of affection she had received caused her to snap, and Marge admits she sympathizes with Lisa, revealing that she locked Homer in the trunk of her car for forgetting their anniversary.

Geriatric Park

In a parody of Jurassic Park, Mr. Burns opens up a Jurassic-themed retirement home, a park filled with elders rejuvenated mixing their DNA with dinosaurs. He brings Springfielders to the park, showing them the new "retirement home". At first they are all human, healthy and feeling young again, with incredible strength too. Once Homer raises the temperature after Grandpa complains about the temperature being too cold with Homer also quoting "I can't read, I'm on vacation" after seeing the warning sign, they transform into dinosaur versions of themselves.

Visitors and Mr. Burns are killed by the elders, Agnes Skinner as a Ludodactylus eats off Principal Skinner's arm, Jasper as a Dilophosaurus bites off Kirk Van Houten's head, and the Simpson family are threatened by Grandpa and Jacqueline Bouvier (turned into an Indominus rex-like theropod and a Parasaurolophus respectively). Lisa courageously confronts Grandpa and find out they all just needed to be cherished and respected. In the end, the Simpson family escape alive and unharmed, though their helicopter is being flown on the back of the transformed Agnes.