A fruit seller is threatened by a protection racket, who obtain false photographs of her lawyer son in an attempt to coerce her into letting them invest in her business on the docks. The racket has not reckoned on the tenacity of Dora and her son, however; or on her good friend Chief Ironside.

Ironside attends a hospital for psychological evaluation, and whilst he is tying knots in his psychiatrist's theories, it transpires that a crook is trying to use the situation to find out the whereabouts of a police informer. With Mark and Eve on holiday, however, there is just Ed to help the Chief, and a whole city to search through for clues.