Reformed safecracker Boston Blackie and his sidekick The Runt are docking at New York harbor upon their return from Europe when Blackie witnesses Marilyn Howard, an attractive fellow passenger, being threatened by a sinister looking man. After coming to Marilyn's rescue, Blackie is greeted by Inspector Faraday of the New York police department who believes that the former thief has stolen the Mansfield pearls, which disappeared shortly before Blackie left for Europe. To obtain a set of Blackie's fingerprints, Faraday places him under arrest and confiscates his landing pass to prevent him from leaving the ship. When Blackie returns to his stateroom to finish packing, he discovers the body of the menacing man, whose name is Martin Vestrick. Knowing that Faraday will accuse him of Vestrick's murder, Blackie pockets the dead man's landing pass, finds The Runt and leaves the boat and Faraday behind. Suspecting that Marilyn is involved in the murder, Blackie follows her to a Coney Island carnival freak show. After instructing The Runt to summon Faraday to his apartment, Blackie joins Marilyn on her ride through the tunnel of love. When Blackie accuses her of murdering Vestrick, Marilyn claims that she killed him in self-defense. Marilyn is about to describe the nefarious organization to which Vestrick belonged when she is stabbed by two men wielding darts. With her dying breath, Marilyn mutters something about the show's mechanical man and warns Blackie to watch the carnival signs. After killing Marilyn, the two men chase Blackie, who runs into the mechanical man's dressing room. When the killers knock on the dressing room door, Blackie slugs the mechanical man and jumps out the window onto the street. Hunted by Marilyn's killers, Blackie hijacks a car driven by Cecelia Bradley and loses his pursuers by driving the car onto a railroad boxcar. Later, when the train stops, Blackie drives out of the boxcar and hears a radio broadcast announcing that Marilyn Howard, an international spy, has been murdered in the tunnel of love. Blackie then introduces himself to Cecelia, who insists on helping him solve the murder. When Cecelia and Blackie arrive at Blackie's apartment, Faraday is waiting to question him about the murders, and Blackie gives him a set of fingerprints to prove that the prints on the gun that shot Vestrick are not his. Satisfied, Faraday leaves and Cecelia and Blackie return to the carnival to investigate. Blackie's alibi backfires, however, when the police lab verifies that although Blackie's prints do not match those found on the gun, they do match prints found on the darts that killed Marilyn. Meanwhile, at the carnival, Blackie is looking through a telescope to investigate some blinking lights emanating from an offshore boat when the mechanical man's thugs spot him. After slugging the thugs with the telescope, Blackie calls his apartment to ask The Runt for help. Faraday is waiting there, however, and after intercepting the message, he ties up The Runt and goes to the carnival to find Blackie. Meanwhile, Blackie disguises himself as a blind man in a wheelchair and watches as the weight guesser misjudges a man's weight by thirty pounds. Realizing that the number must be a signal, Blackie looks up and sees a neon sign flashing in code, and at that moment, Faraday arrives to arrest Blackie. When Faraday refuses Blackie's plea for time to solve the case, Blackie knocks the inspector unconscious, steals his badge and calls the police, claiming that his unconscious friend is Boston Blackie. Blackie and Cecelia then run into the freak show and overhear the spies discuss stealing the plans to a Navy bomb site. Blackie then returns to his apartment just as a police officer and Faraday, who has convinced the carnival police that he is an inspector, enter the building's elevator. After trapping Faraday and his companion in the elevator by turning off the power, Blackie offers to free them if they promise to suspend their chase for one night. They agree but upon releasing The Runt, Blackie locks the policemen in the bedroom and drives with The Runt and Cecelia to her apartment. After deciphering the license plate number on Cecelia's car, Faraday and the officer break out of the bedroom and put a trace on the plate. The next morning, The Runt awakens in Cecelia's apartment to find that Blackie is missing and goes to the boardwallk to find him. There, he is knocked unconscious by the spies. When Blackie returns to Cecelia's with news that he has broken the code and discovered that the spies plan to deliver the stolen plans at seven that night, she insists on accompanying him back to the carnival, so he locks her in the Murphy bed. After Blackie leaves, Faraday, who has traced Cecelia's address through her license plate, arrives and frees her. She then takes Faraday and his police officers to the boardwalk, where he arrests Blackie. When Faraday refuses to believe Blackie's story about the mechanical man and the spies, Blackie throws a ball at the mechanical man, and the barker shoots at them from the stage. Blackie and the police then rush backstage, find the stolen plans and arrest the spies. After Blackie tells the police about the offshore spy boat, he finds The Runt locked in a case used to saw people in half.