The Diving Bell and the Butterball - Homer is paralyzed by a spider bite, but can still communicate...through flatulence. He later becomes a superhero.
Dial D for Diddily - Ned becomes a serial killer by night.
In the Na'vi - Bart gets an alien pregnant when searching for Hilarium.
When Bart, Lisa and Maggie arrive home from trick-or-treating, Marge switches the candy with dental supplies, with Homer owning a bag of candy that Marge wants him to donate. Homer runs away, promising that 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 twenty minutes... Not being able to wait that long, Homer actually 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... it turns out to be vegetables with a short message from Bart and Lisa who took his candy. They are seen laughing and eating the candy. Then, the camera zooms in on Bart's candy-filled mouth, showing the episode's red title.
The Diving Bell and the Butterball
In a parody of The Diving Bell and the Butterfly, Homer is getting Halloween decorations set up. Attempting to find a rubber spider in the decoration box, he accidentally picks up a black widow spider that bites him; this results in his body freezing in an awkward position and him unable to speak. Likely 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; they read off the alphabet and he farts when they get to the letter he wants to form a word with. Then, Homer is bitten by another spider, and turns into a superhero parodying Spider-Man. He then has a dark-blue and red costume and can now fart out spider-webs. Or as Chief Wiggum says "webs".
Ned Flanders is driving through a gritty area of Springfield while giving an opening mental monologue. He briefly encounters a prostitute, but after Ned tells her to "spend less time on your back and more time on your knees", she goes to a Yale School of Prostitution. After a montage of Ned praying, doing house chores, charity work, and disposing of a dismembered man's body (it's unspecified if Ned killed him), Ned witnesses Mr. Burns disposing of toxic waste in a lake. He then hears the voice of God telling him to slay him and urinate in his ashes. Ned is hesitant, but relents and decapitate and dismembers Burns with a knife and throws his head in the radiated lake, causing it to grow a third eye. Ned, under God's instruction, becomes a vigilante serial killer, proceeding to murder Snake, Sideshow Bob (off-screen), and later Patty and Selma Bouvier (in a parody of Wile E. Coyote).
"God" is revealed to be Homer speaking to Ned via a voice changer and a speaker hidden in Ned's Holy Bible. Homer is in the middle of ordering more murders when Bart interrupts, blowing Homer's cover. Ned burst in, enraged at Homer having turned him into a killer, and with the intention of murdering Homer (and presumably himself afterwards). Homer tries to prove that God isn't real by burning Ned's Bible, but then God himself rips off the Simpson's house's roof and strangles Homer to death with his fingers. Marge is horrified and questions God's what happened and God tries to blame Ned for killing Homer. She questions if he can simply make things go back to the way they were. He can, but "the big man downstairs" wouldn't like it. The big man is revealed to be The Devil, who appears and orders God to give him a coffee. Maude Flanders then appears and reveals to be having sex with the Devil, to Ned's annoyance.
In the Na'vi
In a parody of Avatar, Milhouse and Bart are sent to Rigel 7 (the home planet of Kang and Kodos) while in the alien bodies of Rigellians, to retrieve a chemical called Hilarium. A female inhabitant named Kamala screams at Bart, who wonders what she is doing. Kamala states its her seduction voice. Touching Bart's head with her tentacles, Bart is shown an otherworldly vision as he's raped. The Hilarium is found when Kamala's parents say that Hilarium is needed for their pregnant females, or all the females will get grumpy. Then when Milhouse is shown the Hilarium accidentally, he contacts the army that sent him and Bart to Rigel 7.
Later, the army attacks, but the local aliens on Rigel 7 are stronger than the humans, beating them all up. Bart finally gets rid of Superintendent Chalmers by telling him he had something on his cheek while Chalmers was inside a battle suit, causing Chalmers to hit himself and plummet off a steep cliff. Meanwhile, the two aliens, Kang and Kodos, admit that they would have just given the Hilarrium to the humans anyway.
In the final scene, a Christmas carol is played as all of the characters (that appeared in the previous three stories) all said various things about the upcoming Christmas, such as Marge saying "Making a sweater loses 27 people their jobs". At the very end of the carol, Grampa Simpson shows up dressed in a black tutu, asking when the Black Swan movie comes up... everyone was thinking, when the Black Swan will come.
The family in costumes
Bart in his astronaut costume
Dial D for Diddily
In the Na'vi
In the Na'vi 2
Dial D for Diddily 2
The family in costumes 2
Dial D for Diddily 3
Dial D for Diddily 4
This episode is widely considered to be the worst Treehouse of Horror episode due to year old parodies, lame jokes, and lazy storytelling, making it a very weak episode.