Anton Sebastian, the owner of a fabulous hotel and casino known as "The House of a Thousand Candles," greets one of his spies, Victor Demetrius, after he has betrayed his country by obtaining important documents for Sebastian. Sebastian poisons Demetrius, then sends a secret message in the form of a radio broadcast about the hotel's upcoming events to another of his operatives, the lovely dancer Raquel. Sebastian orders her to intercept a British intelligence agent carrying crucial information that could insure world peace. Meanwhile, chief British intelligence officer Sir Andrew McIntyre instructs agent Tony Carleton to pick up some opera tickets, the envelope of which contains the secret information, and then to go to Geneva. Tony gets the envelope and boards a train, but is followed by his persistent American admirer, Carol Vincent. On board the train, Tony meets Raquel, who slips him a drugged drink, steals all of his papers and then escapes to meet Sebastian at the hotel. Carol follows Tony when he jumps off the train in pursuit of Raquel, and although he eludes her when they hitchhike, she reaches the hotel before him and registers them as brother and sister. Sebastian welcomes his new guests, after which Tony goes to investigate, and Carol hides in Raquel's room. She is there when Sebastian plants a microphone, then she goes to his study, where she sees his storage place for stolen papers and listens as he eavesdrops on Tony and Raquel. Raquel is about to tell Tony who she works for when she is killed by her maid Marta, who is loyal to Sebastian. Tony and the precious envelope are then captured by Sebastian, who devises a nefarious scheme whereby Tony is to lose heavily at gambling and then commit "suicide." Carol overhears the plan but is captured before she can seek help. In the casino, Tony signals to his faithful valet Alf to get help from Keith Barrie, a fellow agent who works undercover in the hotel as an orchestra leader. Tony is then brought back to Sebastian and, in order to protect Carol, agrees to decode the information on the envelope. Carol tries to stop him by burning it, but the flames only serve to reveal the information. Sebastian leaves with the message, after which Alf and Barrie rescue Tony and Carol. They find Sebastian's cache of secret codes, after which Tony sets out in pursuit of Sebastian, while Barrie broadcasts a message to Sebastian's operatives telling them to stop a murderer who stole Sebastian's car. Sebastian is forced off the road by his own men, and Tony watches Sebastian's car tumble down a cliff after he retrieves the envelope. Carol and Alf arrive, and Carol follows Tony yet again. Soon after, the newly married Tony and Carol read a newspaper headline about England employing Tony's information to avert war at a Geneva peace conference.