After his sister Susan is arrested in Copenhagen for murdering her partner in a nightclub act, American police detective Mike Brent flies to Denmark to help her. Although there is very strong evidence against her, Susan insists that she is innocent. When Mike contacts Susan’s friend and fellow nightclub performer, Virginia Kelly, Virginia introduces him to her much older lover, Arthur Miller, who offers to assist Mike in proving Susan’s innocence. At night, Mike visits the crime scene and interrupts a thief, Jacobsen, as he steals counterfeit hundred dollar bills concealed in Indian clubs used in Susan’s act. Mike shows the bills to Danish police lieutenant Egon Knudsen, but the lieutenant is already aware of their existence. Jacobsen reports back to Miller, the second-in-command of a gang of counterfeiters, which intends to flood major U.S. and European cities with the bogus currency. Hartman, the gang’s leader, framed Susan for murdering her partner, an errant gang member. Hartman is concerned about Mike’s involvement in the case, although he is unaware that Mike is a police detective. Suspecting that Susan may have been involved with the counterfeiters, Mike accuses her of lying to him and hits her, but she continues to deny any criminal involvement. After Mike establishes that Virginia is unaware of the counterfeit scheme, they become friends. Knudsen attempts to help Mike by arranging for him to meet an informant, but the man is killed before Mike can talk with him. In his hotel, Mike is approached by Lund, one of Hartman’s representatives, who offers to buy back the seventy-two hundred dollar bills Mike found for a meager amount. Mike insists on a dollar-for-dollar arrangement then throws out Lund. After Lund’s body is fished out of the harbor, Mike is informed that he was killed by a gun that Knudsen lent to Mike. Meanwhile, Hartman is determined to proceed with his plan to flood major cities with the counterfeit currency on the same day, three weeks hence and, in order to prevent any interference from Mike, has him captured by gang associate Gibbs and taken first to a boat, then to Miller’s country mansion. Meanwhile, Virginia drives to the mansion where, after telling Miller that she has been interrogated by the police about his activities and that Mike is a police officer, tries to break up with him. When Mike arrives at the mansion, he manages to escape, but Gibbs and Miller chase him through the house. Gibbs finds Virginia and, misunderstanding her presence there, knocks her out. Hartman, intending to double-cross his partners, rigs a car with a bomb with a slow-burning fuse, then orders Gibbs and Miller to drive it to his boat, while he rides there on his motorcycle with the plates for the counterfeit bills. As the police tail the car, it explodes in the middle of a country road. Mike discovers a recovering Virginia in the house and after seeing Hartman leave on his motorcycle, tells her to phone Knudsen. Mike then borrows Miller’s car to pursue Hartman through the streets of a small rural town until he abandons the motorcycle and takes a taxi to the harbor. Meanwhile, Knudsen orders all police units to concentrate on the harbor. Mike, sensing that Hartman is planning to escape from the harbor, reaches the boat first and overpowers the guard. As Hartman steers the boat away from the dock, Mike attacks him and they both fall into the water where they struggle for possession of a knife. After killing Hartman, Mike is picked up by a police helicopter. Later, the destruction of the counterfeit ring results in Susan being cleared of the murder charge, and before returning to the U.S., she and Mike thank Knudsen and Virginia for their help. Virginia, by now romantically involved with Mike, plans to join him shortly.