Tired of being pranked by everyone (his parents, Cosmo and Wanda, his friends, and the rest of the citizens of Dimmsdale), Timmy summons the April Fool, the ultimate prankster, but Timmy does not know that once the April Fool starts, he does not know when to stop. Now, Timmy and his fairies have to stop him.

On Halloween, Timmy wants a Jack-O-Bot costume as his Halloween costume, but the only four Jack-O-Bot costumes in existence were won in a contest, which Trixie, Veronica, Tad, and Chad won because they bribed the judges. To make things worse, Vicky is being paid by Timmy's parents to take Timmy and his friends trick-or-treating, and forces him to wear a mummy costume made out of toilet paper. Vicky gets all of the candy, while Timmy and his friends get nothing. Realizing that Vicky is scary and gets more candy because of it, Timmy wishes that everyone's Halloween costumes were real and scary. This causes everyone to become real and scary versions of the costumes they wear, including the popular kids, who turn into the real Jack-O-Bots and threaten to destroy the world. Also, Timmy cannot undo the wish because Cosmo and Wanda were in costume at the time and are now no longer magical. Meanwhile, the Yugopotamians, seeing people celebrate Halloween, think Earth is planning an attack on them and attempt to counter-attack.