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When Bart, Lisa and Maggie come home from trick-or-treating, Marge switches out the candy with dental hygiene products and asks Homer to deliver the candy to a drive for the troops overseas. Homer instead runs off with the bag to eat its contents in privacy at a canyon cliff. Unfortunately, Homer trips and falls to the bottom of the canyon with his arm pinned.
Fueled by his need to get the out-of-reach bag of candy, and reluctant to wait twenty minutes for help, Homer is forced to chew off his arm. Though he manages it after a few tries, Homer learns that he was carrying a bag of vegetables, as Bart, Lisa and Maggie had secretly stolen back their candy, inciting an upset Homer to scream in rage.
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Homer begins to decorate the house for Halloween and is reaching for a decorations in a box. Unfortunately, there is a real Black Widow spider in the box and when Homer is grabbing for what he thinks is a plastic spider, he gets bitten by the real one. The bite leaves Homer unable to move or talk. When Lisa reads for him, she discovers that he can communicate through flatulence.
The Simpson family is amazed at the result and Lisa helps Homer tell Marge how he feels about her. When he is again bitten by another spider (a radioactive one), he spontaneously gains the ability to make spider webs and have the same abilities as Spider-Man, even when still paralyzed. Homer is able to shoot spider webs out of his rear end.
After hearing who he thinks is God tell him to murder people, Ned Flanders becomes a serial killing vigilante, targeting characters who are Homer's enemies. When Flanders discovers that Homer is the one who has been duping him into committing murder (by way of a Bible-shaped receiver), Homer argues that God does not exist and starts burning Ned's bible only to be stopped and strangled to death by God Himself.
Marge begs God to reverse everything that has happened, but God tells her that Satan is the one who is running the world. When Flanders thinks that this scenario cannot get any worse, it is revealed that Satan is having sex with Ned's deceased wife, Maude.
In the future, Krusty the Clown is still alive, but has trouble entertaining his Nazi audience. On the distant planet Rigel Seven there is a sacred extract called Hilarrium, which causes people to get in a better mood. Krusty desperately needs this sacred extract and uses military force to obtain it. The military recruits Bart and Milhouse to befriend the alien race in avatar bodies. They succeed in this and Bart ends up getting Kang's daughter pregnant.
It turns out that the extract Hilarrium, which is excreted by their queen, is primarily used by the natives to reduce the mood swings from pregnant women and Bart and Milhouse are shown where the alien race gets it from. Milhouse then reveals the location of the extract to the military, which then attacks the natives. In the battle, the natives are helped by all the animals of the planet, which eventually defeats the military. Kang and Kodos then admit that they would have just given the Hilarrium to the humans.
In the final scene, a Christmas carol is played as all of the characters that appeared say various things about Christmas, such as Marge saying that making a sweater causes 27 people to lose their jobs. At the very end of the carol, Grampa shows up in a black tutu, asking when the Black Swan comes up, leaving everyone unsure what to tell him.