Hector Gilligan and his little sister, Cressy, can't wait for the costume party at the new local waterpark. Hector plans to go as the Jellyfish Joker while Cressy dresses up as the famous Crabzilla. But after the Gilligan family returns home, Cressy acts strangely. When they try to take off her costume they can't—because it's the real Crabzilla! How they manage to bring home a giant-sized crab? And where is Hector's little sister?