Famous French scientist Professor Maurice Gaston Le Beau is missing, but a member of the Scientific Section of the United Nations General Intelligence Agency informs three colleagues that he has seen the professor in a sweet shop in the Bowery section of New York City. They all go immediately to Louie's sweet shop, where they find Sach, the professor's look-a-like, and show Slip, Sach's friend, a newspaper clipping, with a photograph, reporting Le Beau's disappearance in Paris. Le Beau, who has been working on a formula for a new rocket fuel, has had several attempts made on his life by foreign agents who want his formula. Consequently, the scientists decide to ask Sach to impersonate Le Beau in the hope that they can force the agents out into the open and allow Le Beau to come out of hiding to resume his research. After the scientists persuade Slip and Sach to accompany them to Paris, Louie decides to go along as well. Soon after, newspaper headlines announce that Le Beau has been found and is suffering from amnesia. On the flight to Paris, Slip fills Sach in on some of the details of Le Beau's life, including the fact that he is engaged to Mimi Du Bois. Meanwhile, in Paris, Mimi informs her friends that Le Beau's physician, Dr. Gaspard, feels that her fiancé can be cured of his amnesia. Unknown to Mimi, Gaspard is in league with a crook named Vidal, who represents a group of agents seeking the formula. When Sach, Slip and Louie arrive in Paris, Mimi and Gaspard accept Sach as Le Beau and indulge his frequently bizarre behavior. Although Gaspard persuades "Le Beau" to restart work in the laboratory, Mimi protests that Gaspard has failed to restore her fiancé's memory and insists that she be given a chance. Mimi then tells "Le Beau" that they will marry during the next week and leave on a honeymoon. After a thug enters the house and throws a knife at him, Sach decides he wants to go home, but Slip convinces him that they are serving both the United States and France by staying. Meanwhile, the real Le Beau, who is cavorting with several maidens on a tropical island, sees a newspaper story about Sach and Mimi together in Paris. Later, when Le Beau returns in a jealous rage and discovers Louie in his house, he orders him to leave. Louie assumes that Le Beau is Sach, gone mad, and is further bewildered when Sach appears and asks him why he is leaving. Confusion reigns as Le Beau and Mimi reunite and he asks her to leave immediately to get married. Slip, too, does not realize that Le Beau has returned. Eventually, Le Beau and Sach meet and Le Beau chases Sach with a sword. When Sach hides, Le Beau starts a sword fight with Slip, whom he regards as another intruder in his house, but Slip locks him in a closet. The confusion is compounded when Gaspard, Vidal and two thugs arrive and demand the formula for the rocket fuel. After Sach dictates a totally arbitrary concoction of ingredients to Slip, Gaspard and Vidal insist that Sach prove that the formula works. Sach then prepares the fuel and puts the mix in a model rocket. Meanwhile, Mimi has freed Le Beau and the thugs bring him to the lab just as Sach lights the fuse on the rocket. To everyone's surprise, the rocket takes off and flies around the lab until it crashes and explodes, wrecking the lab and incapacitating the crooks. Later, the French government awards Sach the Legion of Honor for his accidental discovery, while Mimi consoles the disconsolate Le Beau, who had spent years trying to achieve the same result.
