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American cruise ship Mohawk Prince docks at a London pier, where steward Alfred Gordon asks radio operator Judy Roberts to deliver several packs of cigarettes to a friend on shore. Judy agrees, unaware that Gordon is part of a diamond smuggling ring passing the stones through the cigarette packs. Barely questioned at customs, Judy proceeds to the meeting with Gordon's contact, ex-convict Charlie Williamson, who takes the cigarette packs to a pub where he turns them over to the next contact, Bert, who in turn passes them on to the final courier, Maclure. Maclure reports to the ringleader, wealthy John Hendrick, who is dismayed at the small quantity of stones and demands a larger supply. Meanwhile, at a local pub, Judy meets detectives Dan Barker and his partner, "Mac" McLeod, who cover the river and dock beat. Dan and Judy strike up a friendship and agree to meet the following day at the bar. That evening, Gordon visits Charlie at his home to inform him there must be another delivery the following day. Uneasy because of his criminal past, Charlie attempts to withdraw from the ring, but Gordon forbids it. Later, Charlie lies to his anxious wife Nell, who suspects Gordon of criminal connections. The next day as Judy leaves to meet Dan, Gordon again asks her to carry more cigarettes packs. Although annoyed by the possible delay at meeting Dan, Judy agrees. At the custom checkpoint, however, she is questioned by a guard who inquires if she has paid duty on the cigarettes. When Judy hesitates, the guard examines the packs and, to Judy's horror, discovers several diamonds. While Dan waits in the pub, sitting near an increasingly anxious Charlie and Bert, Judy is held by the police as Blake, the head of customs, informs the Mohawk Prince 's captain, Watford, of the diamond discovery. After witnessing Judy's detainment, Gordon hastily leaves the ship as Watford orders it searched. When Hendrick learns that the police have found the diamonds, he orders Maclure to silence Gordon, who has been reported missing to Watford. Meanwhile on Watford's word, the police release Judy, but order her to remain on board the Mohawk Prince . Disappointed when Judy fails to make their rendezvous, Dan returns to the police station where Mac informs him of the discovery of the diamonds and the search for Gordon. That night on their regular river patrol, Dan and Mac discover in the water a body, which is later identified as that of Gordon. Later, the autopsy indicates that Gordon was killed before being dumped in the river. Dan learns of Judy's involvement and visits the Mohawk Prince to tell her of Gordon's murder and question her. Although affronted at being considered a suspect, Judy agrees to help the police identify Charlie. When the police link Gordon to Hendrick, Dan and Mac take Judy along to question him, but Hendrick claims not to have seen Gordon in several years. Dan returns Judy to the Mohawk Prince , where customs agents planted diamonds in her makeup container. Unaware of the discovery, Dan learns of Bert and Charlie's involvement with Maclure, whom he had questioned earlier. Blake contacts Dan to inform him about the diamonds found in Judy's room, but at Dan's request promises to give the police twenty-four hours before charging Judy. Hoping that Judy can identify Bert or Charlie as Gordon's contact, Dan takes Judy to question them. As Dan questions Bert, he spots Judy waiting outside and denies any involvement in smuggling, then later telephones Charlie to warn him of Dan's impending visit. When Dan and Mac arrive at Charlie's, Nell distracts them, allowing her husband to escape. While waiting outside, Judy spots the fleeing Charlie and, recognizing him, gives chase. To Judy's amazement, Charlie goes directly to the Mohawk Prince and after boarding the ship, she learns from a sailor that Charlie has asked to see Watford. Outside Watford's cabin, Judy overhears his conversation with Charlie, which reveals that the captain is the head of the diamond racket and also Gordon's killer. Judy attempts to contact Dan from the radio room, but Watford stops her. Dan traces the aborted call to the Mohawk Prince and races to the ship. As Judy and Charlie stand by, Watford telephones the police to report that Charlie has murdered Judy. Angered by Watford's audacity, Charlie orders Judy to flee while he attacks Watford. Judy jumps overboard just as Dan's patrol boat arrives. Judy relates Watford's involvement and when they spot Charlie fleeing, Dan and Mac pursue him, leaving Judy in a river boat. Watford slips aboard the boat, knocks Judy out, then attempts to escape. Dan and Mac follow in another boat and when a revived Judy attacks Watford, they are able to catch and subdue him.