The short story deals with a vague and mild-mannered man who drives into Waterbury, Connecticut, with his wife for their regular weekly shopping and his wife's visit to the beauty parlor. During this time he has five heroic daydream episodes. The first is as a pilot of a U.S. Navy flying boat in a storm, then he is a magnificent surgeon performing a one-of-a-kind surgery, then as a deadly assassin testifying in a courtroom, and then as a Royal Air Force pilot volunteering for a daring, secret suicide mission to bomb an ammunition dump. As the story ends, Mitty imagines himself facing a firing squad, "inscrutable to the last." Each of the fantasies is inspired by some detail of Mitty's mundane surroundings:
- The powering up of the "Navy hydroplane" in the opening scene is followed by Mrs. Mitty's complaint that Mitty is "driving too fast", which suggests that his driving is an action of the daydreaming and he has lost touch with the actual world.
- Mitty's turn as a brilliant surgeon immediately follows his taking off and putting on his gloves (as a surgeon dons surgical gloves) and driving past a hospital.
- The courtroom drama cliché "Perhaps this will refresh your memory," which begins the third fantasy, follows Mitty's attempt to remember what his wife told him to buy, when he hears a newsboy shouting about "the Waterbury Trial" ("You miserable cur" are the last words mentioned in the fantasy. Mitty was supposed to buy puppy biscuits.)
- Mitty's fourth daydream comes as he waits for his wife and picks up an old copy of Liberty, reading "Can Germany Conquer the World Through the Air?", and envisions himself fighting Germany while volunteering to pilot a plane normally piloted by two people.
- The closing firing-squad scene comes when Mitty is standing against a wall, smoking.