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Sam Collins, a cab driver, speeds away after hitting a pedestrian, Dr. Norman Selby. Collins is soon arrested and held by homicide inspector Carmichael, who discovers that Selby was shot before he ran out into the street. Carmichael then considers the evidence found at Selby's office: a gun with the serial numbers filed off, a woman's glove and a rare book missing from the doctor's collection. Meanwhile, Selby's fiancée, Marion Gale, not knowing he is dead, prepares to leave the doctor and sail to South America to meet her old lover, pilot Rick Lavery, whom she has not seen in two years. During the cruise, however, the police stop the boat, arrest Marion and bring her back to Carmichael, who tells her that Selby is dead and holds her for questioning. After she identifies the missing book as a valuable Bible, Carmichael suspects notorious gangster Joe Di Lima of masterminding the theft. Joe is, in fact, behind the crime and pays the robber, Louie Gurney, five thousand dollars in exchange for the book. Joe warns Louie not to rat to the police, but Louie is immediately arrested while walking home. Meanwhile, the police also arrest Rick, who has not yet left America, and Carmichael tells him that his own gun killed Selby, and that the men who could serve as his alibi were recently killed in a plane crash. Soon after, a patient of Selby's, Professor Slater, identifies Rick as being in the doctor's office the day of the killing, although Rick denies this and swears that he sold his gun years before. After placing Rick in a holding cell with Marion, the police listen in as they declare both their innocence and their love to each other. When Slater is held in the same room as the couple, Marion realizes that he is nearly blind and so could not have seen Rick in the office. As soon as they are released, she visits his home and forces him to confess that he lied in order to save himself from an earlier, accidental offense. Although he agrees to help her, he is run over by a car on his way to Carmichael's, and soon after, Selby's doorman, Pat Tenny, states that he also saw Rick in the office. Meanwhile, Carmichael releases Louie, who reluctantly returns home, where policemen hiding outside his window stop Joe from killing him. At the police station, Carmichael discovers that the dirt he collected the night before from Tenny's windowsill contains steel filings, and, suspecting Tenny, he visits Rick to propose a deal to him. That night, Rick goes to Tenny and holds him up until the doorman confesses to lying about seeing him at the office, at which point Carmichael steps out of the shadows. He tells Tenny that the steel filings on his window belong to the gun with the filed-off serial numbers, and brings in Louie, who reveals that as he was stealing the Bible, he heard Selby threaten to turn Tenny in for blackmail, and saw Tenny murder Selby. Tenny is arrested, and cleared of all charges, Marion and Rick leave together.