Much to Mugs McGinnis' dismay, everyone in his East Side Kids gang gets to smack his rear end eighteen times in celebration of his eighteenth birthday. His mother Molly then becomes distraught when she gets a letter from his "uncle" Pete, a rancher friend of his late father, stating that he will soon visit them in New York. Molly explains to her only child that ever since his father lied to Pete that he had seven children, Pete has been sending birthday checks for each child. Pete is unaware that the McGinnises are so poor that they could never afford to return the checks. Just then, Pete and his grown daughter Judy ride up to the McGinnis apartment on horseback. Mugs declares that the rest of his supposed brothers and sisters are working at a defense plant, and later, forces his gang to pretend to be his siblings. Glimpy is dressed up like a girl, and Scruno, who is black, is introduced as an adopted child. Pete is delighted by the brood and takes them all out to a nightclub for fun. The next day, local opportunist George Mooney tells Pete that he is being duped by Mugs. Pete is offended when he learns the truth about the McGinnis brood, and tells Mugs to forget he ever had an uncle. The next day, Mugs and the gang go to the hotel to return the gifts and apologize to Judy, and learn that Pete has disappeared. When George, who has arranged Pete's kidnapping, comes to the hotel for a visit, the boys hide in another room, but overhear him say that Pete has had an accident, and that he will take Judy to him. Judy pockets her gun before she leaves with George, and East Side Kid Danny hops onto the bumper of George's car. When the car stops, Danny gets off and calls Mugs to tell him the location of the kidnappers, but is then caught himself and held hostage along with Pete and Judy. The Cherry Street gang joins with the East Side Kids in fighting against the kidnappers, and when policeman Flanagan investigates the ruckus, he arrests the kidnappers. Later, the East Side Kids are guests at Pete's ranch, and try to impress each other by riding bucking broncos.