After learning that her son Lefty has been paroled from prison and is on his way home, Abby "Ma" Dibson, the matriarch of a family of petty thieves, pawns some watches that her daughter Rosalie and Lefty's wife Jessie Belle recently stole from combat-bound servicemen. Keller, the pawnbroker, tries to interest Ma in buying some guns, but Ma turns him down, insisting that they are too risky. While waiting for his older brother at the bus station, Posey Dibson, a convicted pickpocket, is approached by police lieutenant Lorrigan, who frisks him and warns him to stay away from crowds. Later, at the Dibson house, Lefty reunites with his family and is immediately visited by Lorrigan. The detective warns Lefty and Ma that if Posey and Rosalie are caught stealing from servicemen, they will "get the book," then informs Lefty about a job in a dairy. Although Lefty assures Lorrigan and Ma that he will consider the job, he immediately begins plotting to rob McBain, the cash-carrying owner of the bar in which Jessie and Rosalie pick up servicemen. With help from Jessie, Posey then robs two soldiers by posing as the night clerk at a hotel and offering to put their valuables in the hotel safe. When the soldiers realize they have been robbed, they report the theft to Lorrigan and the military police. Although the commanding military officer is anxious to declare the town off-limits to servicemen, Lorrigan reassures him that the police will soon curtail the petty thieves' activities. The next day, Lefty buys Keller's guns and shrugs off Ma and Jessie's threats that they will desert him if he ever uses the weapons. Unknown to the Dibson family, Lorrigan has recruited a group of servicemen to act as decoys and has given them money coated with a fluorescent powder that will rub off when touched. As Posey and Lefty wait for McBain to leave his bar for the night, Jessie hands Lefty a wallet from her latest victim and is soon arrested by Lorrigan, who sees her fingertips glow when exposed to an ultraviolet light. Unaware of Jessie's plight, Lefty robs and kills McBain, then takes his getaway car to be repainted at a garage. When Lorrigan discovers the fluorescent powder on McBain's body, he suspects that Jessie and Lefty are involved. Lorrigan goes to the Dibson house in search of Lefty and announces that he will wait there until Lefty returns. Although Ma tries to alert Lefty and Posey about Lorrigan's stake-out, the crooks are caught by the police outside the Dibson house and brought inside. There, Ma tries to take Lorrigan by surprise while he inspects Lefty's fingers in the semi-darkness, but during a struggle for one of Lefty's guns, Posey is fatally wounded. Lorrigan arrests Lefty, and Lefty, Ma, Rosalie and Jessie are sent to prison.