In 1889, the young Englishwoman Vicky Barton, Jean Simmons, and her older brother Johnny Barton David Tomlinson arrive in Paris to see the Exposition Universelle. They check into the Hôtel de la Licorne, a bustling hotel that feels like a crossroads between elegance and chaos. After a quick dinner, Vicky leads her tired brother to the city’s pleasures, visiting the famous Moulin Rouge before she retires for the night. Johnny, meanwhile, stays out for a late drink, the glow of Paris lingering as a backdrop to a mystery that will soon unfold.
The next morning, the calm shatters when Vicky discovers Johnny is missing and finds a blank wall where his room used to be. His former room number now marks the common bathroom on the floor. Vicky confronts Madame Hervé, Cathleen Nesbitt, the hotel owner, who insists she arrived alone the previous night, a story backed by her brother Narcisse, Marcel Poncin, and the day porter, Eugene Deckers. The tension in the lobby thickens as suspicion circles the hotel’s walls and the memory of Johnny’s presence fades into doubt.
Frantic, Vicky turns to the British consul, Felix Aylmer, who can offer little without a witness and directs her toward any fresh lead. The trail points to Nina, Zena Marshall, the hotel maid who had mentioned plans to go up in a balloon at the Exposition that day. Before Vicky can reach Nina, tragedy strikes—the balloon ascends, bursts into flames, and plummets to the ground, killing the two occupants and silencing a potential witness.
With Johnny’s fate unconfirmed and the authorities slow to act, Vicky faces a mounting wall of indifference. The French police commissaire questions Madame Hervé and her brother but detects nothing suspicious. Because Vicky’s stay was only for two nights, she is given a one-way ticket home to England, a sudden farewell that feels premature and unjust. Yet within this tension, Rhoda O'Donovan, Honor Blackman, whose mother accompanies her, has quietly slipped an envelope under Vicky’s door—an arrangement arranged by George Hathaway to repay a loan to Johnny. The message reveals George’s address and rekindles a stubborn thread of hope.
Driven by a mix of determination and longing, Vicky seeks out George Hathaway. Dirk Bogarde confirms that he met Johnny and offers his help. He studies the hotel’s exterior and notices a striking anomaly: on Vicky’s floor there are six balconies, yet only five rooms remain. Through careful inspection, he uncovers the missing hotel room, its entrance concealed by a wall that has been built over to erase the space.
As pressure mounts, Madame Hervé finally reveals the truth: Johnny had fallen gravely ill on the first night and rang the bell for help. The hotel front desk called a doctor, who diagnosed bubonic plague, and for fear of harming the Exposition’s image, Madame Hervé and the hospital staff whisked Johnny away to a rundown Parisian hospital, effectively erasing any record of his visit. George brings Doctor Hart to the hospital, Felix Aylmer, who examines Johnny and gives Vicky a glimmer of hope—that her brother might still survive. Amid the peril and secrecy, a quiet romance blossoms between George and Vicky, turning their collaboration into a shared resolve to uncover the full story and protect each other as Paris carries on with its grand spectacle around them.
