An unnamed boy, who is the narrator of the book, sits with his sister Sally while home alone on a cold and rainy day, staring wistfully out the window. Then they hear a loud bump which is quickly followed by the arrival of the Cat in the Hat, a tall anthropomorphic cat wearing a red and white-striped top hat and a red bow tie, who proposes to entertain the children with some tricks that he knows. The children's pet fish refuses, insisting that the Cat should leave. The Cat then responds by balancing the fish on the tip of his umbrella. The game quickly becomes increasingly trickier, as the Cat balances himself on a ball and tries to balance many household items on his limbs until he falls on his head, dropping everything he was holding. The fish falls into a pot and admonishes him again, but the Cat in the Hat just proposes another game.
The Cat brings in a big red box from outside, from which he releases two identical blue-haired creatures, or "Things" as he refers them to, in red suits called Thing One and Thing Two. The Things cause more trouble, such as flying kites in the house, knocking pictures off the wall and picking up the children's mother's new polka-dotted dress. All this comes to an end when the fish spots the children's mother out the window. In response, the boy catches the Things in a net and the Cat, apparently ashamed, stores them back in the big red box. He takes it out the front door as the fish and the children survey the mess he has made. But the cat soon returns, riding a machine that picks everything up and cleans the house, delighting the fish and the children. The Cat then leaves just before their mother arrives, and the fish and the children are back where they started at the beginning of the story. As she steps in, the mother asks the children what they did while she was out, but the children are hesitant to answer. The story ends with the narrator asking the reader, "What would you do if your mother asked you?"
