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U.S. Treasury agent Mickey Sweeney is posing as a representative of British criminals who want to sell fake diamonds, which they are passing off as genuine, to an American gang that is eager to convert one million dollars in "hot" currency it has stolen from a Treasury safe. The safecrackers, Karl Hunzinger and his associate, Hoxie, discover Mickey's real identity and kill him. Before he dies, however, Mickey conceals one of the sample diamonds in his mouth. Later, Joe Dennison, Mickey's friend and fellow agent, takes over the investigation and tracks Hunzinger and Hoxie to London where he consults with Inspector Hector "Mac" McClaren of Scotland Yard. At the Yard, two jewelry experts examine the diamond recovered from Mickey's body and estimate its value at twenty thousand pounds. Experts in the Yard's research laboratory then examine the stone and report that, while it appears to be a perfect diamond, it is man-made. Joe also hands over a gun and a special contact lens, used to correct the color of mismatched eyes, found at the scene of Mickey's killing. While at the Yard, Joe bumps into an old girl friend and fellow American, Marlene Miller, who has been recently assigned to TWA's London office. Marlene tells him she is concerned that her father, Dr. Eric Miller, a distinguished scientist, is missing. When Mac, who is Joe's rival for Marlene's attention, asks Marlene to dinner Joe shows up and they both admire a diamond brooch she is wearing. Marlene states that her father made the brooch and cut the diamond himself. Later, Scotland Yard investigators produce a record on Hunzinger, which indicates that the colors of his eyes are mismatched, and after Joe and Mac receive confirmation that Hunzinger is also the owner of the gun, they interview numerous police informants in an attempt to locate him. Later, Joe and Marlene talk with a Chelsea pub owner, named Yeo, who they think may know something about her father's whereabouts, but Yeo denies knowing Miller. When Joe escorts Marlene home, he "borrows" the brooch and takes it to Thompson Blake, one of the precious stone dealers who examined his diamond. Blake confirms that it is identical to the one he saw at Scotland Yard. After Mac runs tests on a pad of paper found in Miller's workshop, he finds the impressions of a word, Tomaszow, which Marlene says is a city in Poland. Marlene is dismayed when she discovers that Joe took her brooch and demands an explanation, forcing Joe to tell her that the police think that her father is mixed up in the manufacture of fake diamonds. Meanwhile, Miller, who has been forced into producing the diamonds, is criticized by Yeo for producing inferior stones. After Miller complains that their original agreement stipulated that they would sell the stones as synthetic, Yeo informs him that a man with a million dollars will soon be arriving to purchase their bogus stones. When Miller refuses to cooperate, Yeo threatens to harm Marlene. Later, Joe and Mac discover that Tomaszow is the name of a Polish tramp steamer, headed toward the English coastline, with Hunzinger and Hoxie on board. After Hunzinger and Hoxie land illegally and are picked up by one of Yeo's men, they are followed by the police in a series of unmarked cars until one car has to swerve to avoid a child in the road and loses the trail. Meanwhile, Yeo delivers the diamonds to his boss, Blake, who leaves to rendezvous with Hunzinger. The exchange is made, but a police constable discovers them and summons Joe and Mac. After Hoxie is captured, Hunzinger runs into a subway station but is shot by Joe, who then recovers the diamonds. Meanwhile, Blake goes to the abandoned castle where Miller has installed giant furnaces in which to produce the diamonds and demands Miller's formula. When Miller stalls, Blake produces Marlene, whom Yeo's men have kidnapped, and has her beaten until Miller delivers the formula, which Blake then proceeds to test. Through the postmark on a letter found in Marlene's handbag, Joe and Mac are able to trace Miller's location. After confirming that Miller's formula works, Blake tricks his men out of their share of the cash, then he and Yeo attempt to blow up the castle by increasing the temperature in the furnaces. Blake then double-crosses Yeo and shoots him. Joe, Mac and the local police arrive in time to rescue Marlene and her father as Blake attempts to flee. Blake is trapped, however, by the dying Yeo who grabs his ankle in a death grip. The others escape before the furnaces explode, killing Blake and Yeo. Joe and Mac then resume their rivalry for Marlene.