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Detective Walsh has been investigating a series of thefts in the jewelry district along New York's Maiden Lane. At the prominent Whitman and Neilson Co., jeweler Neilson discovers that the Montagne Diamond has been stolen and replaced with a look-a-like. The alarm is sounded and Walsh responds. Whitman explains that after he showed the diamond to Frank Peyton, he noticed that it had been switched. When Payton is questioned, however, he denies knowledge of the theft. Walsh also questions a young woman named Jane Martin, into whose purse Payton slipped the diamond. Whitman is worried, but he tells Walsh that he pays a high premium to Jewelers' Indemnity and Protective Corporation. At the office of John Graves, the corporation's president, Nick Shelby, acting as representative for the thieves, tells him that he can recover the stone for $6,000, far less than it would cost to replace it. At the front desk of her apartment building, Jane places the stolen diamond into her strongbox for safekeeping. She then goes up to her apartment where Peyton, who has been waiting for her inside, searches her purse for the diamond. She tells him where she has hidden it, and intrigued by her behavior, Peyton asks her why she did not tell the detective that he had planted it on her. She replies that she too is a thief, but in reality, she is Graves's niece and is bent on capturing the thieves who have cost her father so much money. Peyton suggests that they work together to dispose of the diamond and arranges a meeting place with Shelby to collect the money. While Peyton is distracted, Jane secretly calls the police and tips them off to the meeting. However, before the police arrive, Peyton has already left the scene with Jane. Suspicious of Jane, Peyton stages a robbery in which a man is apparently shot to see if she will remain silent. When Peyton tells her that the robbery was a test and that the man did not die, Jane decides that she must prove beyond a doubt that she too is a thief. Enlisting her uncle in the scheme, Jane pretends to steal an emerald necklace to demonstrate her skill. Convinced, Peyton invites her to the home of Gilbert Lockhart, one of the biggest jewelry brokers in the city. There, Lockhart shows a film demonstrating the cutting of the famous Magna Diamonds, worth over a million dollars. Lockhart reveals that he has contracted to purchase the diamonds from the owner, Harold Anderson, who is in attendance. After the party, Anderson is abducted by one of Lockhart's men, robbed of the million dollars and murdered. Meanwhile, Walsh, who has learned Jane's true identity, realizes that she is in terrible danger. He rounds up his men, and they travel to Lockhart's house, where they arrive just in time to capture Peyton and Lockhart. Once she has recovered from a gunshot wound she received from Lockhart, Jane vows to give up her amateur detecting, and she and Walsh have lunch.