Young Robert "Bibi" Bonnard, Bobby Driscoll, grows up in Ottawa, Ontario, under the wide-eyed gaze of his parents, Jacques Bonnard and Susan Bonnard. Across the street, his roving grandfather, Grandpere Bonnard, Marcel Dalio keeps a watchful, if mischievous, eye on the family’s everyday lives. The neighborhood also features his amiable uncle Louis Bonnard, Kurt Kasznar, a genial drunkard who tends to overlook the struggles of his hard-working wife, Felice Bonnard, Jeanette Nolan, and their worried daughter Yvonne Bonnard, Annabel Faber. Louis eagerly contemplates meeting his potential son-in-law, Alfred Grattin, Richard Erdman, a teetotaler bank clerk who hopes to marry Yvonne. Nearby, Peggy O'Hare, Marlene Cameron, is a neighbor and schoolmate who carries a quiet crush on Bibi, even as he is not quite sure how to feel.
On Bibi's birthday, the boy is taken to a theater where his violinist and conductor father works, and the magical world of vaudeville unfolds. During a high-stakes act, the renowned illusionist The Great Gaspari, Maurice Marsac, attempts a kiss on his assistant Mignonette Chappuis, Linda Christian, who is in the middle of a death-defying sawing-in-half routine. Frustrated by the scene, Mignonette storms off and quits, and Jacques offers her a job as a maid, a proposal she takes with a mixture of caution and curiosity. Into this moment steps Uncle Desmond Bonnard, Louis Jourdan, a traveling salesman and notorious ladies’ man, who has been summoned back to fill a temporary role at the family’s side. Desmond’s charm quickly returns to the forefront as he pursues Mignonette, who, despite feeling drawn to him, longs for a more settled life. He teases her with visions of a house he expects to inherit, slowly weakening her resolve.
Meanwhile, Peggy’s jealousy over Bibi’s attention to Mignonette intensifies. Bibi—already a bit of a misfit at school—has slipped into trouble for bringing La Vie Parisienne to class. When a dirty picture is found by the school principal, Mr. Frye, Jack Raine, Peggy falsely accuses Bibi of drawing it. Bibi denies the accusation, inciting Frye’s punitive response: three firm taps on the hand, a threat of repeating the punishment daily until he confesses. The Bonnards intervene, confronting Frye and striving to set the record straight, a process that proves to be anything but easy.
As the adults regroup and return home triumphant over the immediate crisis, Desmond learns that Mignonette has quit after discovering he lied about the house and suspects him of creeping into her room at night. Desmond’s assumptions crumble as Bibi confesses that he is the true culprit, and Desmond begins to see that Mignonette is not like the other women in his history. He locates her again, and the two decide to become engaged.
In the aftermath, the adults explain Peggy’s behavior to Bibi, and, to Peggy’s delight, he forgives her and makes her his girl. Yet, just as happiness pivots on the edge of growing up, Bibi’s voice betrays the-changing moment with a crack that whispers of changing times and new responsibilities.
