While in Paris to deliver an address to the French Psychiatric Institute, Dr. Ordway, the renowned criminologist, meets his old friend, Paris Chief of Police Jacques Morrell. Morrell insists that his friend dispense with all thoughts of business while they spend an evening reveling in the Paris night life. When they visit a nightclub featuring the knife-throwing act of Maurice Duval and his daughter Mignon, Morrell ominously remarks that he believes that Duval may have used his talents for murder. The next day, Morrell entices Ordway into helping interrogate Henri Jardin, a young man accused of murdering his father. Under Ordway's questioning, Henri recalls the events the preceded his father's murder: Having just married Mignon against his father's wishes, Henri is urged by their mutual friend, artist Anton Geroux, to reconcile with his wealthy parent. Following Anton's advice, Henri goes to speak with Jardin, who upon hearing the news of the marriage, disowns his son, resulting in an acrimonious fight after which Henri can remember nothing. After Henri is dismissed, Morrell explains that the young man was imprisoned in a concentration camp during the war, and after his release, suffered a mental breakdown. Morrell then states that he believes that Maurice stabbed and killed Jardin with one of his knives. Proceeding to the Jardin house, the scene of the murder, Morrell and Ordway meet Jules Daudet, the family attorney, who plans to enter a plea of insanity on Henri's behalf. Ordway next confers with Maurice at the Duval apartment. When Maurice opposes his daughter's marriage, Ordway reminds him that Mignon stands to inherit the Jardin fortune if Henri is judged both guilty and insane. Ordway's next stop is Anton's studio, where he realizes that Anton is in love with Mignon, a possible motive for framing her husband for murder. As Ordway is about to leave, Anton offers to sell him some copies of masterpieces that he has painted and asks the doctor to meet him at antique dealer Louis Chabonet's shop later that evening. Before his meeting with Anton, Ordway visits Mignon in her dressing room to enlist her help in proving Henri's innocence. Meanwhile, on the dark street outside Chabonet's shop, Anton is accosted and murdered by a shadowy figure. When Ordway arrives, the figure knocks him unconscious and then disappears. The police believe that the intruder was a burglar, but are puzzled when Chabonet declares that nothing has been disturbed. When they discover that copies of Anton's are missing, they assume that the thief mistook them for the originals. Later, at the Jardin house, Daudet shows Morrell and Ordway an old contract that he has just found involving Maurice and Jardin. In the contract, dated fourteen years earlier, Jardin loaned Maurice money, using some property as collateral. According to Jardin's butler, Jardin refused repayment of the loan and foreclosed on the property, leading Daudet to propose that Maurice murdered Jardin out of revenge. When Morrell goes to arrest Maurice for suspicion of murder, however, he finds him in bed, dead, with a suicide note confessing to Jardin's murder on his nightstand. With Maurice's confession, Henri is released from custody and joins Mignon at home. Soon after, Ordway visits the couple and declares that he thinks the suicide note was a hoax because the coroner's report indicated that Maurice died of natural causes. Ordway then asks for Henri's help in exposing the real murderer and directs him to put all his father's personal effects up for auction. On the day of the auction, two Americans, Brown and O'Reilly, come to bid on a valuable painting. When O'Reilly exposes the canvas as a forgery, thus dashing Henri's hopes of raising a nest egg from the auction proceeds, Daudet expresses sympathy for his young client and makes a nominal bid for the reproduction. The night the painting is delivered to Daudet's office, Ordway sneaks into the building and watches as Daudet picks up an ax and is about to swing it at the work. When Ordway stops him, Daudet hurls the ax at him and a fight ensues. After a struggle, Ordway overpowers Daudet and summons Morrell. Ordway then reveals that he staged the auction to force the killer to bid on the painting because once it was exposed as a forgery, the person responsible for it would want it back. Ordway continues that Daudet killed Jardin because Jardin saw him switch his original for Anton's copy and then he killed Anton to silence him.