The title, in British police parlance of the day, defines a petty crook whose criminal activities are minor and legal-borderline, but the title character in this film appears to stretch the definition. Benny steals a purse from Melissa Stribling and discovers a compromising letter in it from her boyfriend Colin Tapley, who is a wealthy surgeon and married. Benny starts a blackmail campaign that leads to murder. Susan Shaw is along as Benny’s girlfriend, Molly.