Steve Buchanan, a trouble-shooter from the Los Angeles office of the Secret Service branch of the Treasury Department, is called to New York to help with the case of Mister "880," the file number of a counterfeiter of one dollar bills, who, though ridiculously inept, has eluded the department for years. As he investigates at a cigar store, an elderly junk salesman known as "The Skipper" Miller buys tobacco but stops himself as he is about to pay, and takes out a bill from a different pocket. The Skipper returns to his apartment building, where he sells his neighbor, Ann Winslow, a miniature spinning wheel. Delighted with it, she insists on paying five dollars, despite his insistence on a price of three dollars, and while she is not looking, he slips two dollars into into her purse. When an angry veterinarian threatens to see his lawyer if the Skipper does not pay him money owed for his dog's treatment, the Skipper promises to visit his rich "cousin Henry." He then uses "Henry," the name he uses for his primitive counterfeiting apparatus, to produce a number of new bills. When Ann is identified as the person who passed a counterfeit bill, Steve and his partner Mac follow her to the lobby of the United Nations, where she buys a newspaper with another phony bill. After she speaks with a man there, Steve bumps into him to get a good look at his face. He learns that Ann is a UN interpreter, and although he does not believe her to be Mister 880, he decides to "cultivate" her for information. When she stops to look at a display in an art gallery window, Mac's attempts to flirt become a nuisance, and Steve and Mac pretend to fight. Steve then has drinks with Ann and makes a date with her for the next evening. The next day, Ann returns to the art gallery and learns that after the incident the previous day, Mac identified himself as a Secret Service agent. She calls the Secret Service office, asking for Steve, and when he answers, she hangs up. Realizing that Steve arranged to meet her to investigate a case, Ann, who is attracted to Steve, fears that he will soon lose interest in her if he learns that she is not a counterfeiter. When he comes to her apartment that evening, she leaves an incriminating letter in her typewriter, then later in the evening, uses some counterfeiting slang that she picked up from a book in the library. Steve informs her that the phrase she has spoken has not been used since the Civil War, then reveals that she has passed two counterfeit dollar bills. He believes her when she says she has no idea where she got them, and she is relieved when he says that he will hold her in "technical custody." Discerning a behavior pattern of the counterfeiter, Steve stations Mac and himself at various places of business where phony bills have been passed, but the Skipper passes a bill right in front of Mac, because of the Skipper's unassuming behavior. Steve soon realizes that the counterfeiter has passed bills at Coney Island every year on the same Sunday. Still not knowing that the Skipper is the counterfeiter, Steve convinces Ann to change her plans of going to the zoo with the Skipper and the neighborhood children and join him at Coney Island. Although a hawker who was given a phony bill identifies Steve as the counterfeiter, Steve feels that they are on the right track and has cards passed out in Bay Ridge and Canarsee telling people how to identify counterfeit bills. After the Skipper passes a phony bill at a fountain in Bay Ridge, he notices the card, then returns home and buries "Henry." After some time passes and no new bills are passed, Mac asks to be taken off the case. Steve's boss in Washington wants to send him to France on a new case, but Steve, consumed with the desire to solve this case, refuses. Meanwhile, Ann learns that the Skipper has sold all his "antiques" for money to live on and arranges for him to be a handyman in the building. Steve reveals that he has decided to take the assignment in France and asks if she can get a trip to Paris, but she warns him of leaving a task undone. When the Skipper sells Ann a matching miniature spinning wheel, he confesses that he earlier left the two dollars in her purse, and she realizes that he is the counterfeiter. Later, when Steve tells her that he has declined the Paris assignment and is back on the 880 case, Ann worries that the Skipper will be apprehended and sentenced to fifteen years in prison. After a boy is caught passing phony money in a grocery store, Steve traces the bill back to a boy who located "Henry" and, thinking the bills he made were stage money, traded some to friends. Steve now realizes that the Skipper is the culprit, and finds more incriminating evidence in his room. When a distressed Ann tries to convince Steve that the Skipper would not survive a long incarceration, Steve is unmoved. The Skipper then arrives with food and wine to celebrate his new job, and when Steve reveals that he is with the Secret Service, the Skipper admits that he has passed counterfeit bills and agrees to come to the station for questioning. There, the Skipper says that he never gave out more than one bill at a time, and always bought something, so that the victims of his crime would not suffer too much from the loss. He also reveals that by counterfeiting he has been able to afford to live outside of the Naval veterans home, which would have cost the government more than the losses from the counterfeiting did. The New York office chief, angry at the embarrassment the Skipper has caused, wants to prosecute him to the fullest. At the trial, the Skipper refuses counsel, and insists on pleading guilty. Steve's testimony is pivotal, and though he says that counterfeiters should be dealt with as severely as possible, he does not think that the Skipper committed his crime because of greed. After the Skipper's exemplary Navy record is discussed, the judge sets the sentence at the minumum, and when he reveals that Skipper can get be paroled in four months, Ann kisses Steve in appreciation.