A European vacation turns a teenaged girl into a woman.

While on holiday in the champagne country of France, four English children are stranded when their mother becomes ill and is hospitalized. The oldest child, 16-year-old Joss, takes her sisters, Hester and Vicky, and brother Willmouse, to the château-hotel on the River Marne where their mother had reservations. At first the proprietor, Madame Zizi, and the receptionist, Madame Corbet, are reluctant to take the children in, but Zizi's lover, a dashing Englishman named Eliot, insists that they be received. The next few days are filled with delight for the children as Eliot takes them sightseeing and dining. Zizi, however, resents sharing Eliot and becomes suspicious of his attentions to the rapidly-maturing Joss. One night Zizi creates a scene in the dining room by throwing a glass of champagne into Joss's face; but when Eliot follows Zizi to her bedroom, the outraged Joss gets drunk with the scullery boy, Paul, and has to be carried to her room. The next day, in a fit of adolescent jealousy, Joss sends Eliot's photograph to the Paris police, having correctly concluded that he is a wanted jewel thief. That night Paul comes into Joss's bedroom and attempts to rape her; and as Eliot, roused by the terrified girl's screams, rushes into the room, Paul falls from a window to his death. Ashamed of her actions, Joss confesses to having mailed the photograph. Though Eliot makes his escape, his concern for the children is such that he sends a wire to their uncle, summoning him to the château, thus divulging to the police his whereabouts. Before leaving the château, a much wiser Joss recalls Eliot's parting words: "Little Joss, in this summer you grew up. You've become a woman."