Tom's chasing Jerry around Mammy the maid, when she accidentally clobbers Tom with her broom. This makes Tom think he's a mouse, including crowding into Jerry's hole and eating cheese. Neither Jerry nor Mammy are happy about Tom's amnesia. Jerry tries to hit Tom again to change him back, but seems to have lost his aim (even a piano misses). A bowling ball does catapult Tom into the ceiling and change him back, but then on the rebound, he gets hit again and changes into a mouse again. Finally, a Rube Goldberg sequence triggered by Jerry's carelessly tossed bit of cheese drops an ironing board on Tom's head. Success! Except that Mammy, who's reached the same conclusion as Jerry, happens along with a baseball bat and changes Tom back into a mouse.