Mary Turner, an impoverished salesgirl at Edward Gilder's department store, is sentenced to a three-year prison term for a theft she did not commit. As she is taken from the courtroom, Mary lashes out, saying that someday she will even the score with the merciless Gilder and the law. While in prison, Mary is befriended by the more worldly Agnes “Aggie” Lynch, who suggests they see each other after they are released. At the end of her sentence, Mary is unable to find work and goes to Aggie for help. Although Aggie does not have much money to spare, she introduces her to a friend, Joe Garson, an amiable career criminal, who is attracted to Mary but does not want anything to do with a girl who is not “a regular.” Joe instead offers to bring her into his racket. She declines, but convinces him that there is a legal way to make a lot of money by enticing wealthy men into writing love letters, then having their lawyers pay them for their return. Soon, with Joe, Aggie and Joe’s cohort Red, Mary makes a fortune with variations on the scheme. Although they are extorting money, Mary insists on having legal documents for everything, making their activities technically within the law. As part of Mary’s plan for revenge against Gilder, she begins seeing his son Bob, who falls in love with her. One day, Sgt. Cassidy, a detective who has been after Joe, comes to Mary’s apartment and demands $30,000 in bonds that they had received from one of their wealthy victims. Joe is worried, but when Mary comes home, she insists the bonds were legally acquired and produces a restraining order to keep Cassidy from harassing her and Joe. After Cassidy leaves, Mary begs Joe not to lose his head because, if he is convicted of another crime, he will be sent to prison for life. Later, Cassidy relates what has happened to his superior, Inspector Burke, who promises to place Joe and Mary behind bars. Sometime later, on Bob’s birthday, he and Mary arrive at his father's mansion and announce that they are married. Gilder at first does not recognize the now sophisticated and expensively dressed Mary, but when he does, warns her to leave Bob alone. Although Mary lies to Bob that she does not love him, he realizes that she cares and vows to stay with her. Meanwhile, Burke has arranged for “Dapper” Eddie Griggs, an old cohort of Joe, to approach him about a deal. Eddie tells Joe that the Mona Lisa , which had been stolen from the Louvre museum but recently recovered, is actually a fake and the real painting is hanging in Gilder’s mansion. He convinces Joe that they can steal the real painting and sell it to a fence for $200,000. Mary warns Joe not to become involved, but he thinks it will be easy money that will enable him to get away from Cassidy. On the night of the robbery, Joe, Eddie, Red and another cohort enter Gilder’s mansion through a manhole cover after tying up some electrical workers and taking their coveralls. Because of the ruse, Burke and his men, who are watching nearby, do not recognize them. Later, when Mary learns that Joe is going through with the robbery, she also enters the house. Inside, she sees Bob, to whom Burke had lied that Mary left town. Realizing that Burke has been using them all, Mary exposes Eddie as an informant. Joe then shoots and kills Eddie. When the police storm the house, a shootout ensues, during which Red is shot but Joe escapes. After Burke finds Bob and Mary inside, Bob coolly tries to divert attention from Eddie’s body, but when the corpse is exposed, Burke accuses Bob of murder. Mary immediately tells them that Bob was within his legal rights to kill a burglar. At police headquarters, both Bob and Mary are put through the third degree, but they do not change their stories, even when Burke tricks them by allowing them time alone in a room with a hidden microphone. After Red dies refusing to finger his friends for Eddie’s murder, Burke’s men capture Joe. Although Mary begs Joe not to admit to Eddie’s murder, he confesses so that Bob and Mary can be together. After his confession, Eddie asks Bob to take good care of her.