Mary Carner, the efficient department store detective, leaves her job at Blankfort's Fifth Avenue Store when her friend, Phyllis Knight, a young socialite attorney is found murdered after having been missing for six months. Inspector Heinsheimer of the New York Homicide Squad admires Mary Carner and is willing to work with her, but Mary is finally on her own entirely; poking into the affairs of Rockey Nardello who is doing time as leader of a numbers racket gang. Dangerous? So Mary Carner found out!