It is 1916 after the beginning of the First World War, in London. A somber group is filing past a casket surrounded by candles. The butler sees everyone out, locks the door, then takes down the casket, which is empty. Next we are at a British intelligence office in the middle of an air raid, and an officer in uniform comes in, holding a newspaper that seems to announce his own death. The officer, a writer by profession, (John Gielgud), is given false passports with the name Ashenden, introduced to an assistant (Peter Lorre) who goes by the nickname General, and told to proceed in disguise to Switzerland, Hotel Excelsior, and await further instructions. The instructions are to identify a German undercover agent who is at the hotel and about to leave Switzerland for Istanbul, then to prevent the German agent from reaching his destination by any means necessary. In Switzerland we see an old man buy a bar of chocolate, open it, and read a hidden message that a British spy is expected to arrive at the hotel Excelsior. Ashenden checks into the hotel and learns that Mrs. Ashenden (Madeleine Carroll) had arrived the day before. He goes to the rooms reserved for the couple and finds Mrs. Ashenden in conversation with an American, Mr. Marvin (Robert Young), who has been insinuating himself to her. The fake Ashenden couple have their initial conversations to get acquainted, and the General comes in also. Ashenden and the General go to the church to meet the organist, a supposed double agent, to get more information, but they find the organist dead at the organ, with a special type of button in his fist. Back at the hotel they strike up a conversation with a British tourist, they all go to the casino together, and are surprised when the torn button appears to come from the British tourist's coat sleeve, supposedly identifying the tourist as the killer of the organist. The group, including Marvin, have dinner at the same table, and meet the wife of the tourist, who is German and says this trip is the first time she has ever been out of Germany. Ashenden and the General make bets about which of them is a better mountain hiker. They convince the British tourist to go up a nearby mountain the next day, where they plan to murder him. Ashenden has misgivings in participating in a murder, and tries to turn back from the climb, but the General continues up the mountain and pushes the tourist off a cliff. Back at the hotel they receive word that they have identified the wrong man and thus murdered an innocent Briton. This causes consternation and conflict between the Ashendens, he taking the view that he was on an assignment in wartime, she that the murder of an innocent is too high a price to pay even in wartime. This moral conflict is a continuing source of much anguish for both of them, who are otherwise much attracted to each other.Meantime, the General has snooped around and found that the chocolate factory is a center for spy messages, and talks Ashenden into going there to try to get more information from the boyfriend of one of the hotel maids, who has a job at the factory. Mrs. Ashenden decides she wants to resign from her spying assignment and separate herself from her fake husband, and, having no special place to go, she decides to go with Marvin on the next leg of his train trip. At the chocolate factory Ashenden and the General are fingered as British spies and the police is called. They stage an emergency and pull a fire alarm to create chaos, and succeed in evading the police and returning to the hotel, where they find Mrs. Ashenden gone (accompanying Marvin) and also get a coded telegram that identifies Marvin as the agent. They manage to get to the train station, warn Mrs. Ashenden that Marvin is their target, and she agrees to resume her spy duties by pretending to be romantically interested in Marvin. They get on a train that eventually crosses into neutral territory and then into another country at war against Britain. As they confront Marvin in a compartment and he admits being a German agent, British planes attack the train, producing a train wreck. Marvin is mortally wounded, the General is shot dead, and Mr. and Mrs. Ashenden, now in love with each other, are able to return to England without having had to physically murder anyone. A brief epilogue follows, showing the triumph of British forces in the Eastern (Turkish) front.
Output: Question: Who concocted a bet?, Answer: Ashenden and the General