The episode begins with a parody of Tales from the Crypt, with Mr. Burns as the Crypt Keeper. The scene begins in a dungeon room, where a crypt opens, and after several waves of rats, snakes, spiders and rabbits crawl out of a coffin, the Cryptkeeper sits up in it. He proclaims himself to be the master of "scare-amonies" to the delight of zombie Smithers. A bound Moe interrupts in protest and is killed in an iron maiden, his blood spilling onto the floor and spelling out "Treehouse of Horror XVII". Moe himself takes delight in this and proclaims "A Ho Ho! Look at that, my blood is a genius! Fancy Roman numerals and everything!"
In a parody of The Blob, as Homer and Marge make out in the backyard, a meteorite falls nearby, burning off the top of Marge's hair in the process. It cracks open to reveal a green gooey substance that resembles a burning marshmallow. Homer, despite his family's objections (and the goo's attempts to flee), puts it on a stick and devours it. Later that night, his stomach rumbles from hunger, and Homer eats all the available food. He also eats Snowball V and then he attempts to eat Bart but is stopped by Marge. Homer then becomes morbidly obese, and eats a teenager in an attempt to "savor" him from the flames of a barbecue fire. Homer slowly mutates into a giant blob monster, rampaging through the streets of Springfield, eating all the overweight people he can find. Homer snacks on bus passengers as if they were chocolates and decapitates Ned Flanders. Dr. Phil McGraw shows up with the Simpson family and tells Homer to stop for their sakes. Homer ends his rampaging for fear of losing Marge and vows to use his insatiable appetite for more constructive purposes (he eats Dr. Phil anyway; his last words are "Food does not equal love!" before he digests into nothingness). Mayor Quimby later dedicates a new homeless shelter. The homeless people enter the shelter, only to find themselves inside Homer's gut.
In a parody of The Golem, at the end of an episode of Krusty's show, Bart goes backstage to complain about an acid-spraying Krusty brand alarm clock. There he finds the Golem of Prague, a creature from Jewish mythology. Krusty tells Bart that in the 17th century, the Golem was sculpted out of clay by a powerful rabbi and would obey any command written on a scroll and placed in his mouth. Although the golem was created ostensibly to protect Jewish villages, he would obey any scroll placed in his mouth, evil or good. He had been passed down through many generations and now works for Krusty (primarily to deal with hecklers). Bart steals the Golem by writing a command for him to come to his home at midnight. At midnight, the Golem shows up at the Simpsons' house. From then on, Bart uses him to carry out his commands: swinging Principal Skinner up and down like a yo-yo until he splits in half, and kicking Homer's walls (the result of a misunderstanding, as the Golem cannot read Bart's handwriting). Lisa thinks the Golem does not like doing the biddings of others and feeds him a scroll reading "Speak". The Golem (voiced by Richard Lewis) attempts to roar, then coughs, and reveals that he is a decent being who feels guilty about being used to commit heinous acts, and then he throws up excessive scrolls, one of which reads "Kill the Czar". To make him feel better, the Simpsons create a female Golem (voiced by Fran Drescher) out of Play-Doh. The two are married by Rabbi Hyman Krustofski and the female Golem convinces Chief Wiggum not to press charges on Skinner's murder with the promise of pan-fried latkes, a Jewish delicacy (though she only gets to the words "pan fried" before Wiggum agrees).
The population of Springfield are fooled by Orson Welles' infamous The War of the Worlds 1938 radio broadcast and believe the world has been invaded. Mass panic breaks out, and the citizens begin rioting. Marge voices her belief that the Martians will only destroy humans, so Sideshow Mel suggests they foil the aliens by cavorting naked in the mud like animals. They do this until the following day, when Lisa tells the citizens that it was all a hoax. Angry at being fooled, the citizens of Springfield vow to never fall for such a trick again. Meanwhile, Kang and Kodos, observing the entire event from their orbiting spaceship, decide this is the perfect time for a real invasion, and begin destroying what is left of the town. True to their word, the town does not believe that it is a real invasion and ignores it. Orson Welles comes to Springfield and admits it is not a staged act, but is unable to rouse the disbelieving citizens. The segment ends by jumping forward to three years later, with Kang and Kodos looking over the ruins of occupied Springfield and mulling on what went wrong and why they were not greeted as liberators, as they planned the invasion to rid Earth of "weapons of mass disintegration" which they refer to as "Operation Enduring Occupation" (in a reference to the 2003 invasion of Iraq). The segment ends with the camera pulling away from the smoking ruins of what was once Springfield, as the song "I Don't Want to Set the World on Fire" by The Ink Spots plays.