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.
In a parody of The Diving Bell and the Butterfly, Homer begins to decorate the house for Halloween and is reaching for a decoration in a box. Unfortunately, there is a real Black Widow spider in the box and when Homer grabs what he thinks is a plastic spider, he is bitten by it. The bite leaves Homer paralyzed and unable to speak. When Lisa reads for him, she discovers that he can communicate through farting. Lisa is amazed at the result and helps Homer tell Marge how he feels about her. When he is again bitten by another spider (a radioactive one this time), he gains the ability to shoot spider webs out of his rear end and have the same abilities as Spider-Man, though he is still unable to move or talk.
In a parody of Dexter, after hearing a voice that 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 sleeping with Ned's deceased wife, Maude.
In a parody of Avatar, taking place in the future, Krusty the Clown sends the military force to the planet Rigel 7 to find the a sacred extract Hilarrium, so he can easily entertain his Nazi audience. 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. They learn that the extract Hilarrium, which is excreted by their queen, is primarily used by the natives to reduce the mood swings of pregnant women. Milhouse then contacts the military of their location, 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.
At the end, 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, Grampa Simpson shows up in a black tutu, asking when the Black Swan comes up, and everyone wonders when the Black Swan will come.