Oscar Lemon, a bumbling Swedish immigrant fond of practical jokes, lives in Orange, New Jersey with his adoring sister, Hilda Blake, his less hospitable brother-in-law and their daughter June. Oscar is fired from his job at a toy shop for taking too much pleasure in amusing his juvenile customers with gag tricks. Meanwhile, gangster Silent McGee, a dead ringer for Lemon and a master of disguise, plans with his henchman, Smithy, to steal a truckload of liquor belonging to rival gang leader Pierre La Rue. During the heist, Oscar passes the crime scene and is mistaken for McGee, the first of many such mix-ups. Moments later, La Rue's limousine is showered with bullets, and his sister, the lovely Julie La Rue, vows to avenge her brother's death. Julie and Tony, La Rue's right-hand man, encounter Oscar walking down the street and believe that he is McGee disguised as a Swede. Julie lures Oscar to the Golden Slipper, a speakeasy where she sings, and tries to seduce him into revealing the whereabouts of the truck before bumping him off. Failing to procure the information, Julie tries to coerce a now drunken Oscar into a phone booth, the appointed place for the hit. Instead, Oscar wanders into the ladies' dressing room, where he swallows a miniature harmonica and subsequently squeaks whenever squeezed. Julie finally extorts with kisses and pleas the truck's whereabouts that Oscar, who saw after it broke down near his home earlier, innocently reveals. Jerry, June's boyfriend and a junior reporter, arrives at the Golden Slipper and warns Oscar about the gangsters' error in identification. Oscar, finally realizing what is going on, tries to disguise himself and then escapes through a trapdoor in the phone booth only to learn that Julie has been taken hostage by McGee's men pending McGee's return. Oscar, now consciously assuming his disguise as McGee, rescues Julie with Jerry's help, just as the real McGee arrives. After a riotous chase, the doubles meet on the Blakes's front porch for a final duel. The police arrive and offer Oscar a $10,000 reward for McGee's capture, and Julie, relieved that Oscar is not really McGee, squeezes her brave Swede, whose swallowed harmonica squeaks one last time.