When babysitter Lulu takes the five little monkeys trick-or-treating, they decide to change costumes with their friends and try to fool Lulu and their mother.

"Claudia's not worried when she hears her newest babysitting charge, Betsy Sobak, is a great practical joker. After all, how much harm can one little girl do? Plenty. Claudia breaks her leg as a result of one of Betsy's mean jokes--and now she's talking about quitting the club. Babysitting, she says, is just too dangerous. Kristy thinks Betsy needs to be taught a lesson. She also thinks the Babysitters are just the ones to do it. Watch out, Betsy...