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.
- While diving at a pier, a boy finds the body of a young woman. The main clue detectives have to work with is a heart-shaped tattoo on her hand with the letters "MAC" inside. Lieutenant Steve Carrella brings his wife Teddy along as he talks to one of the many tattoo parlor owners in hope of finding clues to the killer. After this visit, Teddy decides to get a new tattoo herself---and inadvertently winds up also following the killer.