In Los Angeles, an informant who has promised to turn over a sample of the paper being used by a counterfeiting ring to a U.S. Treasury agent is killed. Later, in Washington, Treasury Department chief Carson assigns Dennis O'Brien and Anthony Genaro to infiltrate the gang through connections in Detroit. O'Brien and Genaro assume the names Vannie Harrigan and Tony Galvani, respectively, and create personal background histories to support their new identities. After the Detroit police cooperate by identifying them as robbery suspects, the agents contact with local crime boss Carlo Vantucci, who runs a produce market as a front. Vantucci checks them out and hires them to work in his counterfeit liquor stamps racket, where they soon discover the name of the mob's Los Angeles connection, The Schemer. After Dennis leaves for Los Angeles, Vantucci roughs up Tony, but Tony covers for his fellow agent. A long, laborious search through Los Angeles pays off when Dennis locates The Schemer in a bath house. Dennis then contacts local Treasury chief Gregg, who supplies him with some counterfeit bank notes, and follows The Schemer to a hotel, where they both become involved in an illegal dice game. Dennis passes a counterfeit bill, which The Schemer immediately recognizes and offers to supply him with better paper if he will provide the engraving plates. After Dennis follows The Schemer to the Club Trinidad near Pacific Ocean Park, The Schemer makes contact with a club photographer, who takes the phony bill to her boss, Paul Miller, at a photo lab. When The Schemer finds out that Dennis has been following him, he has him mugged by two of his men, Moxie and Brownie, who later take him to The Schemer's boss, Shiv Triano. As they play pool, Dennis suggests a partnership to Triano. Later, Triano checks out Dennis with Vantucci, who also sends Tony to Los Angeles. Dennis sends a sample of the paper to the Treasury Department's lab in Washington and learns that it may have been made in China. After Dennis delivers one of the engraving plates, to Triano, he makes a deal to deliver the other when he meets Triano's boss, the mastermind of the whole organization. Dennis and Tony then try to set up The Schemer, who reveals to them that he has written down, in code, all the activities of the local "Mr. Big." While Tony and The Schemer are shopping at the Farmer's Market, Tony is recognized by a friend of his wife, Mary, who is accompanying her on a visit from San Francisco. Mary, suddenly realizing that she must not reveal their relationship and identity in front of a stranger, declares to her friend that she is mistaken about Tony being her husband and leaves. The boss, meanwhile, has arrived on a ship from China and Dennis meets with his second-in-command, Diana Simpson. Lacking confidence in The Schemer, Diana orders Moxie to eliminate him, but before Moxie kills him in a steam bath, The Schemer tells him that Tony has been trying to make phone calls to San Francisco. After Triano has his people in San Francisco check on Mary, he tells Dennis that his friend is a Treasury agent. Just as Tony locates The Schemer's notebook, Triano shoots him in front of Dennis and Tony dies without implicating him. Los Angeles agents then gain possession of the notebook, and the simple code is broken, revealing details of profits from gambling, liquor and counterfeiting activities. Dennis is warned that the Department has discovered that Miller, the gang's engraver, can identify the source of the face plate as he worked with the man who made it. Dennis is advised to abandon the case but persists, and although the boss rejects the plate for some reason, Dennis insists that they bring Miller to the tanker. There Miller claims not to recognize the plate but privately tells Dennis that he does, then realizes that Dennis is a Treasury agent and offers to turn government witness. However, their talk is overheard by Moxie, who kills Miller. Dennis takes Miller's gun and, after a chase during which he is wounded, kills Moxie. The police arrive in force and swarm over the ship, finally arresting the ringleader, whose associates in Detroit and Shanghai are also picked up. Dennis recovers and Mary Genaro carries in her heart the memory of her husband who died in the service of his country.