This is the 25th Treehouse of Horror in the Simpsons organized by Matt Groening. It is divided in three parts. Fox also said that one of the main characters will die in season 25.
Bart looses his head and then finds it attached to Lisa's. Together they go to second grade and at night Bart finds out that when Lisa sleeps he is in control of her body.
The Simpsons kids are sick and can't have candy or go treek or treating, but then comes the Fat in the hat (who is basicly Homer) and takes them to the streets. Later Bart, Lisa, and Maggie try to escape because Fat in the Hat wants them to stay with him forever. A parody of cat in the hat of Dr. Seus's rhyme books.
At the long time ago circus Strongman Homer wants to take the devil creaters's (Moe's) emerald, so he makes Marge marry him to later kill him, but Marge doesn't want to kill Moe, so she dumps Strongman. Meanwhile, a group of circus clowns and monsters kill Mr.Burnso and take over the circus.