Mammy Two Shoes clocks Tom on the head with her broom, scrambling his brain, making him think that he is a mouse.
A very silly but also very amusing short, Nit-Witty Kitty sees the usually cunning cat believing that he is a huge rodent: he becomes buck-toothed, squeaks, and hops around the house in the search for cheese. He also crams himself into Jerry's mouse hole (ooeerr!), which makes the mouse extremely fed up, especially when Tom breaks his bed. Reading up on the subject, Jerry discovers that another bang on the bonce might set Tom straight and reaches for his baseball bat. Unfortunately, Mammy also has the same idea
What follows is a series of attempts by Jerry to knock some sense into Tom using such items as a baseball bat, a flat iron, and a bowling ball, but to no avail; the one time he is successful at reverting Tom to his feline ways, Mammy enters and bashes Tom back into his mouse persona. It's all very daft, but also very funny.