It’s almost Halloween. Assuming that Principal Malcolm doesn’t know what Halloween is, they offer to teach Creepschool what Halloween is all about by throwing a party for all the monsters. In their search for costumes, the kids come across an armoire full of great costumes they can use for the party. Just after the party starts, a terrible storm breaks out and the lights go out. The guests cry out and when the lights come back on, everyone has vanished except for the four kids. Believing their friends to be in danger, the kids set off looking for them. They soon discover that they cannot remove their costumes, that they have in fact become what they got dressed up as. This comes in handy, however, as they need the powers that come with their new identities in order to overcome the obstacles they come across along the way. In the end it turns out that the monsters were playing a trick on them- in fact, they invented Halloween so there would be one day in the year when the monsters could walk around the city without having to wear disguises.