In 1889, young Englishwoman Vicky Barton (Jean Simmons) and her brother Johnny (David Tomlinson) arrive in Paris to see the Exposition Universelle. This is Vicky's first time in Paris, and after checking into a hotel, ...展开she drags her tired brother to dinner and the famous Moulin Rouge. She finally retires for the night, while Johnny has a late-night drink. When English painter George Hathaway (Dirk Bogarde) drops off his girlfriend, Rhoda O'Donovan (Honor Blackman), and her mother (Betty Warren) at the hotel, he asks Johnny for change for a 100 franc note to pay a carriage driver; Johnny loans him 50 francs and gives him his name and room number.
The next morning, Vicky finds a blank wall where Johnny's room used to be. When she questions hotel owner and manager Madame Hervé (Cathleen Nesbitt), the latter claims she arrived alone. The room number now adorns the common bathroom. Madame Hervé's brother Narcisse (Marcel Pontin) and the day porter (Eugene Deckers) back up her story.
Frantic, Vicky goes to see the British consul (Felix Aylmer), followed secretly by Narcisse. She has no proof of her brother's existence, so the consul can only suggest she find a witness, Nina (Zena Marshall), the hotel maid who attended her. Nina had informed her that she was going up in a balloon with her boyfriend at the Exposition that day, so the consul takes her there. Tragically, she is too late. Before she can talk to Nina, the balloon ascends, bursts into flames, and plummets to the ground, killing the two passengers.
Vicky tries the French police commissaire (Austin Trevor). He questions Madame Hervé and her brother, but can find nothing amiss in their story. Since her room has been reserved for only two nights, Vicky has to leave the hotel. Madame Hervé offers her a ticket home to England, which she is forced to accept, as she has little money left. However, unbeknownst to either party, Rhoda O'Donovan has been asked by George Hathaway to deliver a letter containing his loan repayment to Johnny. Not finding his room, Rhoda slips the envelope under Vicky's door, where she finds it.
Vicky goes to see George. When he confirms having met her brother, she bursts into tears. He offers his assistance. George notices there are six balconies, but only five rooms on the floor, and finds the missing hotel room, the entrance having been covered over to be part of the wall.
Under questioning by the police, Madame Hervé reveals where Johnny has been taken. It turns out that he became sick with the Black Plague during the night. The news would have been disastrous for the Exposition, so he was secretly taken away to a hospital. George brings along Doctor Hart (André Morell), who tells Vicky her brother has a chance of living.