Captain Dandy Forsdyke, James Robertson Justice, is a habitual criminal whose talents lie in pickpocketing and safecracking. He never uses his real name, a secret he guards with shame. He is engaged to Babette LaVern, a stripper who wants him to go straight before they marry. In love with Babette, he longs to quit, but old associates keep pulling him back into crime. Babette LaVern Julie Christie encounters a society called Crooks Anonymous that helps hardened thieves go straight. Founded by Laurence Montague, Wilfrid Hyde-White leads the group, funded by a generous legacy, and they have an excellent track record. Babette asks them to help Forsdyke reform.
Forsdyke is picked up during a robbery by Brother Widdowes, Stanley Baxter, a Crooks Anonymous man disguised as a policeman, and taken to the organization’s headquarters. Confronted by Montague, Forsdyke confesses that he wants to quit crime and marry Babette. They begin to interrogate him, and discover that he is a habitual liar, whose real name is Cox, who has never seen military service, despite his claim to be a decorated veteran.
Widdowes and Montague embark Forsdyke on punishment therapy and rehabilitation. They begin by locking him in a room filled with safes, which contain cigarettes, food, drink and a number of booby traps that make opening them a hazardous business. After a week of this torture, Forsdyke is beginning to crack. Nonetheless, he fails a test to see how much progress he has made, reverting to his old ways as soon as he is outside.
After a month of correctional therapy and reinforcement training, Forsdyke finally passes the test and is released into society. He moves into a house with Babette, gets a job working as Father Christmas in a department store and refuses an offer by one of his old pals to go back into criminality.
However, after consuming large amounts of alcohol at a Christmas party he passes out and finds himself alone in the department store, with £250,000 of takings near him in the safe. Forsdyke breaks in intending to steal the money, and then panicking at his relapse, calls Crooks Anonymous for help. They send their two top men, including Brother Widdowes, who also cave at the sight of the money. They in turn call for Senior Brother, who arrives with his Secretary.
Confronted with this unique opportunity, the five of them decide to steal the money and split it among themselves. They go to Forsdyke's house, only to be confronted by an outraged Babette, who demands they put the money back. Grudgingly they agree, as she threatens to call Scotland Yard and inform them of the burglary.
They successfully manage to return the money, unfortunately triggering an alarm which brings the night watchmen out. Forsdyke marries Babette and as a sign of appreciation they make her a Guardian Angel of Crooks Anonymous, for keeping them all honest.
