Frederick and Eleanor Little are somewhat surprised at the birth of their second son to find that he is hardly bigger than a mouse. In fact, he looks quite a bit like a mouse with whiskers, a tail, and a pleasant mousy manner. In only a few days, he even acts like a mouse, wearing a tiny hat and carrying a walking stick. The Littles name him Stuart.

Mrs. Little weighs Stuart daily on a postal scale, and when Stuart is a month old, his parents send for a doctor to see why he has gained only a third of an ounce. The doctor is unconcerned to find a mouse in the family and assures Stuart’s parents that he is perfectly well.

Stuart is a lively, helpful boy. He doesn’t mind doing those jobs for which only someone his size is particularly suited. One day, Mrs. Little loses a ring down the bathtub drain. Stuart’s older brother George suggests that she try fishing the ring out of the drain with a pin on a string, but she is unable to catch hold of it.

Mr. Little suggests sending Stuart down the drain, and Stuart is willing to have a try. He slides easily into the drain and finds the ring, and his father pulls him back up by a string.