The Cats go inside a museum before closing time and accidentally are locked inside it. They fall asleep among the exhibits, but are wakened by a reanimated Egyptian mummy, who's after a valuable ruby in one of the display cases. They realize that they'll have to catch the runaway mummy or they'll be blamed for the theft of the ruby. They set several traps to stop the mummy in its tracks and retrieve the ruby. Finally, the mummy is revealed to be a famous jewel thief in disguise who snuck into the museum in the mummy case to steal the jewel.