The body of a young woman is found by a boy diving from a pier. The coroner finds that she died, not by drowning, but due to arsenic poisoning. The main clue in the detectives' investigation is a small heart-shaped tattoo on her hand with the letters "MAC" inside of it. Detective Steve Carrella takes his wife Teddy along with him to a tattoo parlor as he questions the owner. Later, Teddy decides to get a tattoo herself. In doing so, however, she has put herself in grave danger as the killer has come to the same tattoo parlor with his intended next victim.