The opening has Homer Simpson with a bag of candy that Marge wants him to donate. Homer runs away, promising he won't eat it. After driving many miles he reaches "Candy-Eating Peak." He runs and climbs up it until he falls into a hole, and gets his right arm trapped under a boulder. He calls 911, who say they'll be there in 20 minutes. Not being able to wait that long, Homer tries to eat his arm off. He eats his left arm and leg off, then finally eats his the right arm off and opens the bag and it's turns out to be vegetables with a message from Bart and Lisa who took his candy. They are seen laughing and eating candy. Then, the camera zooms in on Bart's mouth, showing the episode's title.
At the end, a Christmas carol is played as all the characters that appeared say various things about Christmas, such as Marge saying that making a sweater loses 27 people their jobs. At the very end, Grampa Simpson shows up in a black tutu, asking when the Black Swan comes up, and everyone becomes nervous.
Homer is paralyzed by a black widow spider and is unable to speak, hinting his case is locked-in syndrome. While Lisa is reading a long book to Homer, Homer tries to tell Lisa he doesn't want to listen, and starts farting repeatedly. Lisa figures out that Homer can communicate this way. Then, Homer is bitten by a spider from the Springfield Nuclear Power Plant, and turns into a superhero parodying Spider-Man. He then has a dark blue and red costume and can fart out spider webs. Or as Chief Wiggum put it, "Ass webs".
Ned Flanders is tricked by Homer to kill by Homer using a voice warper to pose as God. Ned kills many of Homer's enemies, starting Charles Montgomery Burns after Ned sees Mr. Burns dumping radioactive waste into a lake. Homer tells Ned to turn Mr. Burns to ashes, then pee on the ashes. Ned then cuts Mr. Burns' head off and throws it into the lake, and Mr. Burns' head grows a third eye. Ned also kills Sideshow Bob and Patty and Selma. Patty and Selma are lured by free cigarettes, then killed, parodying Wile E. Coyote but without failure. Ned figures out Homer made him into a killer when Homer says hi to Bart. Ned then appears behind Homer. Homer starts burning Ned's Holy Bible that he had under his arm when suddenly the Simpson's roof is torn off and the real God chokes Homer to death. Marge comes in and asks if God can fix everything, but God says the Devil wouldn't like it. The Devil appears with Maude, and it is shown that God is under the Devil's power when the Devil asks for a coffee.
Milhouse and Bart are sent to Rigel 7 (the home planet of Kang and Kodos) to retrieve Hilarium. Bart impregnates a local inhabitant named Kamala on the way. The hilarium is found when Kamala's parents say that hilarium is needed for their pregnant females, or they will get grumpy. When shown the hilarium, Milhouse contacts the army that sent him and Bart to Rigel 7. The army attacks, but the animals on Rigel 7 are stronger, beating them all up. Bart finally gets rid of Superintendent Chalmers by telling him he had something on his cheek while Chalmers was in a battle suit, causing Chalmers to hit himself and plummet. This section of the episode was based on the movie Avatar.