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Silk, a safecracker who is part of a gang of thieves, successfully steals the LaPaz diamond, but is double-crossed by gang members Phil and Mamie, who steal the diamond and go into hiding. Months later, the case remains unsolved by the police, and Phil and Mamie arrive in town with the diamond and an infant boy. Posing as the Duke and Duchess of Leradia, Phil and Mamie take a room at the Massey Arms, which is managed by Charlie Moore, whom Silk knows. One evening, detective Russ Ashton agrees to watch the baby and guard the royal couple's possessions and leaves his dimwitted assistant, Harvard, in charge. Silk's accomplice, Maxine, drugs Harvard's coffee and switches the diamond for a fake. Silk gives the real stone to Moore for safekeeping so that "Diamonds," the gang boss, does not get his hands on it. Harvard revives and, finding the baby playing with the fake diamond, pockets it. When he spills milk on his suit, Harvard calls his fiancée Veronica for a change of clothes, and Susie, Russ's secretary and girl friend, exchanges the clothes. Veronica, finding the diamond in the suit, assumes it is a present from Harvard. When Russ forces her to return it, she tells the policeman on the beat, Officer Murphy, that he tried to accost her, and Russ involves police lieutenant "Mac" McGruder in the case. Russ returns the fake diamond to Phil and Mamie, who pulls a gun on him before Mac enters and arrests them. Mac is now eager to arrest Harvard, but he is being held hostage by Diamonds and his gang. When he tells them Susie took the suit, Diamonds and his henchman abduct Veronica and Susie. Although they have been ordered to guard Harvard, Silk and Maxine tie him up and visit Moore, who has informed them that the police are in Phil and Mamie's room. Harvard escapes and helps Russ search the room as Maxine enters to retrieve the spoon with which she stirred Harvard's soporific drug. Harvard chases her and is held up by Silk and Moore. Russ rescues Harvard and retrieves the real diamond, then he and Harvard rescue Susie and Veronica just as Diamonds' henchman is about to kill them. Later, Veronica tells Russ and Ashton that their reward money might buy her a diamond for her engagement ring.