In the summer of 1938, Austrian medical student Peter Vadassy is vacationing at the Hotel Reserve on the French Riviera when he goes to the village to pick up a roll of film that he has left to be developed. After the nervous clerk claims that the film is not ready and asks Vadassy's name and address, two policemen enter the shop and demand that Vadassy accompany them to the police station. There, Michel Beghin of French Naval Intelligence produces Vadassy's Austrian passport and questions his allegiance to France. After Vadassy protests that he has renounced his Austrian heritage and applied for French citizenship, Beghin exhibits the roll of film that contains photographs of secret French fortifications outside of Toulon and accuses him of espionage. When Vadassy realizes that his camera had been switched, Beghin speculates that the real spy, a guest at the hotel, took the pictures. Threatening Vadassy with deportation unless he exposes the photographer, Beghin hands him the guest list and sends him back to the hotel. At the Hotel Reserve, Vadassy scrutinizes the list and envisions each suspect: Suzanne Koche, the Germanic proprietess; the talkative Robert Duclos; newlyweds Andre and Odette Roux; Henri Asticot, an enthusiastic sports fisherman; Mary Skelton, a charming girl he met at the beach; Walter and Hilda Vogel, a middle- aged couple; and Emil Schimler, a sinister German. At dinner that evening, Andre notices that Vadassy is nervous and sends him a glass of champagne. When Vadassy declines the drink and tells the waiter to offer it to Schimler, who is seated in a corner, the waiter responds that the man's name is Heinberger, not Schimler. After retiring to his room that night, Vadassy overhears Madame Koche advise Schimler to be patient. The next day, Vadassy eagerly visits Beghin to confide his supicions about Schimler. In response, Beghin becomes angry and orders Vadassy to stop playing spy and ascertain which guests own cameras. Back at the hotel, Asticot catches an enormous fish and Vadassy uses the occasion to urge the guests to fetch their cameras and photograph the catch, noting that Schimler protests that he has no camera. Later that afternoon, Vadassy goes to his room to don his swimsuit and go bathing with Mary. Upon entering the room, he is struck from behind by an unseen assailant who has been rifling his drawers in search of the film. Deciding to use the theft to flush out the spy, Vadassy tells Duclos about the intruder, knowing that he will spread the story about the burglary. When Duclos gossips about the theft, gleefully increasing the scope of the crime as he speaks to each guest, Madame Koche summons Vadassy to her office and angrily orders him to leave the hotel the following day. Desperate to uncover evidence, Vadassy sneaks into Schimler's room and discovers that he possesses three passports, each bearing a different name. When Schimler returns to his room and discovers Vadassy, he threatens to call the police. After Vadassy challenges him about the three passports, Schimler admits to being a member of the anti-Nazi underground and is now being hunted by the Nazis. Later, as a storm rages outside, the guests congregate in the lobby, where Vadassy notices a camera identical to his own dangling from Andre's pocket. When Vadassy announces that nothing was actually stolen from his room, the irritable Andre verbally attacks Duclos as a gossip and then storms up the stairs to his room. There, Odette accuses him of bungling the job of securing the strategic photographs. Just then, Vadassy knocks at the door and asks to borrow a roll of film. After Odette quickly hides the camera, Andre invites Vadassy to join him for a drink in the lobby. At the bar, Andre admits that he accidentally switched cameras with Vadassy and offers to buy the photographs. Schimler walks past them just as Vadassy declines the offer. As Andre pulls out his gun and demands the negatives, the police arrive and arrest Vadassy for espionage. At the police station, Beghin presents Vadassy with his French citizenship papers and explains that he was using him to trick Andre into revealing the location of the spy ring operating in the area. Meanwhile, at the hotel, Schimler is in the midst of disabling Andre's car when Andre and Odette enter the garage and Andre shoots and kills him. As Andre speeds through town and onto the coastal road leading to Toulon, Beghin and Vadassy follow. After Andre turns into the spies' headquarters at a deserted warehouse along the docks, Beghin and his men charge up the stairs to arrest the ring. Andre wounds Beghin and climbs out a window, but Vadassy pursues him across the rooftops, driving him to the ledge, from which Andre plunges to his death. With his name finally exonerated, Vadassy returns to the hotel to join Mary for a swim.