When his wife dies during childbirth, intern Bob Morley becomes disconsolate and disappears for ten days before returning to his worried Aunt Hilda and angry Uncle John Miller, a German-American pet shop owner, to tell them he is going away. The Millers offer to rear the boy, whom Bob refuses to see. Bob goes to Vienna for four years of intensive study with a leading surgeon. Two years later, when Hilda reads that Bob, now a child specialist, is about to be married, she brings his son Billy to his office. Bob is quite moved and breaks a lunch date with his fiancée Martha to attend Billy's sixth birthday party. When he is late for a reception, Martha finds him at the Miller's pet shop. Upset at what she calls the "horrible neighborhood" and "common people," Martha leaves in a huff after Billy expresses displeasure at her interference. When Bob explains that Billy is his son, they reconcile. After they marry, Martha suggests that they get Billy to live with them. John angrily refuses, but a judge rules in Bob's favor because the Millers never legally adopted the boy. Although Billy cries profusely in Hilda's arms and refuses to go with Bob, when Hilda insists, he obeys. Billy's initial days at the Morley home are trying for all concerned. Billy is uneasy with the servants and his domineering French tutor, Mam'selle, while Martha, upset at Billy's lack of table manners, also fears that Bob loves Billy more than her. When Bob encourages her to endear herself to Billy, she agrees to try and sends Mam'selle to buy Billy a dog, but when he sees the small "prize" dog she selects, he calls it a rat and cries. After the Millers visit with Beans, the mutt Billy loved when he lived with them, and Jiggs, a chimpanzee, Billy hides Beans in his bedroom closet. That night, after Bob is called out of town for an operation, Martha's friends, Paul, Gladys and Frank, visit for bridge, while Jiggs comes to Billy's window and joins Billy and Beans in devouring a cake Hilda baked. Although Martha does not get angry, Mam'selle yells at Billy. After Gladys and Frank leave, Paul flirts with Martha and encourages her to leave Bob. He attempts to kiss her when Billy, running away, sees them and tries to help Martha. Paul pushes him down and his head hits the floor hard, which causes him to go into a deep coma. When Bob returns, Martha, who has given Billy a lot of blood and attended to him day and night, packs because she feels that the incident was her fault. However, as Billy starts to revive, he calls Martha "mother," which moves her deeply and she embraces Bob. Sometime later, they happily celebrate Billy's birthday with the Millers and Billy's friends and animals.