Arthur, Brain, Binky, and Buster are all pranked by an unidentified prankster. Brain mistakes jelly worms in his lunch for real ones, Arthur finds a fake spider in his locker and Binky finds a skull. Each prank comes with an invitation and they are invited to the library, where Muffy wears a fake tentacle and pretends to strangle Francine, which frightens them.
Arthur asks what the whole thing is about and Muffy says she's having a costume party for a new book. E.A DePoe, alias Ms. McWord, Ed Crosswire's old teacher from The Scare Your Pants Off Club, will be reading her book aloud at the party.
Muffy also says the boys will be her "extra special" guests, due to a prank the boys pulled on her on April Fool's Day. The party will be held in Castle Manor, which is supposedly haunted (which is why the party is being held there).
The girls come up with a prank that involves glow-in-the-dark balloons and a tape recorder, while the boys decide to turn the tables on the girls. They come up with several prank ideas centered around popular tales (The Tell-Tale Heart, The War of the Worlds, and The Legend of Sleepy Hollow), but they can't decide on which one to do.
Ultimately, Arthur comes up with a prank that involves Binky wearing a pumpkin head and crab claws and standing behind a curtain, so when the girls see his shadow (courtesy of Buster with a flashlight) making eerie sounds (performed by The Brain), they'll have their pants scared off. The boys agree to use Arthur's idea.
During the party, Arthur tells Buster and Brain they can set up in the upstairs library (which is where the girls' prank is). However, Binky is nowhere to be seen.
A child in a sheet and wearing a pumpkin shows up at the punch bowl next to Arthur, who assumes that the child is Binky. Arthur tells him to go up to the library and meet up with Buster and the Brain.
Arthur gets Francine and Muffy to come up to the library where the girls try to pull their prank, but they discover that the tape recorder is gone. Suddenly, the pumpkin-head kid appears in front of them and then vanishes, and the girls run screaming.
The girls are upset and accuse Arthur of stealing their prank, but Arthur calls for the boys to come out. Buster and The Brain arrive and tell Arthur that the prank has been moved to the playroom because the library was locked.
Arthur points out that the prank went perfectly, but Brain says they didn't do it. Buster points out that the balloons and tape recorder from the girls' prank were in there, but Francine didn't move them. The pumpkin-head child appears, and Arthur asks for an explanation.
Suddenly, Binky enters in a cyclops costume because he broke his pumpkin, and Arthur is now bewildered. After asking "Binky, if you just got here..." Arthur turns to the pumpkin-head child and finishes with "then, who is-?"
When they look down and see that the pumpkin-head child is floating off the ground, the gang realizes that the pumpkin-head child is really a ghost.
The children run downstairs and try to explain what happened, but Ed doesn't listen and scolds them about pulling pranks. In the attic, the ghost turns out to be the ghost of a little girl who was only trying to go to the party until she got caught up in the pranks.
Her ghost father also scolds her for pulling pranks, saying that there are other ways to have fun. The episode ends with the gang putting off pranks for the time being, while the ghost family secretly attends the party.