Vicky Barton visits Paris with her brother Johnny, only to discover the following morning he has gone missing and the hotel staff have no recollection of his presence.

May 1889. Like many others, young-adult siblings Johnny and Victoria Barton, British nationals who have only each other in the world, have arrived in Paris, traveling from Naples via Marseilles, for the World's Exposition. While Victoria is a bundle of excitement about their 48-hour stay in Paris, to include the opening day of the Exhibition, Johnny is preoccupied by all the logistics of their extended vacation. The one day changes 180 degrees when first thing in the morning, Victoria cannot only not locate Johnny, but there is no indication that he was ever at their hotel. The hotel owner Mme. Hervé, her brother Narcisse the front desk clerk, and bellboy who waited on them deny that anyone accompanied Victoria to the hotel, his name is not in the hotel register, and even his hotel room, number 19, has totally disappeared--Mme. Hervé shows Victoria that room 19 is only a lavatory. Beyond speaking to the British Consul and the police, who can do nothing to help her until she comes up with some proof that Johnny not only exists but was accompanying her, a panicked Victoria tries to find anyone, especially at the hotel, who can corroborate that Johnny was with her. She might have found that someone in George Hathaway, a fellow British national living in Paris eking out a living as an artist; friends of his are also staying at the hotel for the Exhibition. Victoria and George together try to discover exactly what happened to Johnny and why Mme. Hervé and others at the hotel are lying about his existence.—Huggo

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, 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 friend, 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 lends 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 Poncin) 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. 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 commissioner (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 bubonic 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.

Vicky Barton (Jean Simmons) and her brother Johnny (David Tomlinson) take a trip to the 1889 Paris Exhibition. They sleep in separate, adjoining rooms in a hotel. When Vicky gets up the next morning, Johnny and his room have disappeared and nobody will even acknowledge that he was ever there. Now Vicky must find out what exactly happened to her brother.—DarkDan <[email protected]>