Joss Grey Susannah York, a sixteen-year-old English girl, must look after her three younger siblings during a summer in France after their mother is suddenly taken ill and rushed to the hospital.
When they go to the Hotel Oeillets, the proprietress Mademoiselle Zisi Danielle Darrieux does not want the responsibility of unchaperoned children, but her enigmatic English lover Eliot Kenneth More persuades her to accept them.
As the days pass, she wishes she had stuck to her original answer; she becomes increasingly jealous of the attention Eliot pays to the children—especially to Joss.
Meanwhile, hotel employee Paul becomes suspicious of Eliot, snoops in his room, and finds a pistol.
Eliot catches Paul and gets Zisi to fire him, but Joss's thirteen-year-old sister Hester Jane Asher has taken a liking to Paul and begs Joss to get Eliot to reconsider, which he does. But later he becomes angry when Hester takes his picture. Then he rushes out of a tour of caves where champagne is stored to avoid famous guest Monsieur Renard Raymond Gérôme, the best policeman in France. He also insists on turning away potential guests.
Tensions come to a boiling point when Zisi throws a glass of champagne in her rival's face. Eliot chases after her, saying—within Joss's hearing—that she is only a child. Learning from a newspaper article that Eliot is a notorious jewel thief, the outraged Joss mails Hester's photo of him to the police.
Eliot has already decided to leave. He sneaks out late at night, but, on hearing a drunken Paul attack Joss in her bedroom on the second floor, he rushes up to her room. He punches Paul, who then tries to climb down a drainpipe, but the pipe breaks and Paul falls to his death. Eliot tells everyone not to call the police. A remorseful Joss confesses to Eliot that she has denounced him to the police. At her request, he gives her a grownup kiss. Then he disposes of Paul's body and disappears.
While Renard is questioning the uncooperative children the next morning, their solicitor uncle, Mr Bullock Maurice Denham arrives. He has been summoned by an unsigned telegram to extricate them. From the source of the message, Renard realizes that it is from Eliot and that he is trying to escape across the border to Germany on a river barge. Renard explains that Eliot is now also suspected of murdering Paul, but Joss states he died in an accident after trying to escape from her room. With it now almost certain that the police will capture Eliot attempting to escape via the river barge, Hester breaks down in tears and is consoled by her uncle, while Joss walks away alone from the hotel down a country lane, disconsolate.
