On the Bombay Mail, the train between Calcutta and Bombay, Sir Anthony Daniels, the British governor of Bengal, is murdered. Traveling on the same train are his wife, Lady Daniels; his two private secretaries, Capt. General Worthing and William Luka Patson; John Hawley, rushing to register his ruby field; the Maharajah of Zungore; Dr. Maurice Lenoir; Pundit Chandra, an anti-British rebel; a woman claiming to be a Russian opera singer; the mysterious Xavier and several Americans. Inspector Dyke of Scotland Yard joins the train at its next stop to investigate the murder. Before Daniels died, Capt. Worthing had inquired whether he was going to be charged as a spy because of his relationship with the Russian woman, but Daniels would say nothing. When the governor's body is found in the compartment shared by Pundit Chandra and John Hawley, Dyke learns that Daniels was killed with cyanide. Several people on the train had access to the poison including Hawley and Lady Daniels. The compartment adjoining Chandra and Hawley's belongs to Beatrice Jones, a Canadian. Lady Daniels tells Dyke that Daniels received an anonymous letter denouncing Jones as someone who should be deported. At the next stop, the maharajah is asked to accept a salute, and during the honors, he is shot and killed. Meanwhile, someone steals the rubies that Hawley was bringing with him to Bombay. Dr. Lenoir, whose speciality is poisons, is traveling with a king cobra. When Hawley accuses Xavier of stealing his jewels, Xavier opens the bag to prove his innocence and is bitten by the snake. Hawley then accuses Beatrice of stealing the rubies. She tells Dyke that she had been singing with a Russian opera company and, finding herself stranded, sent the anonymous letter, hoping to be deported. When this failed, Capt. Worthing gave her his government pass, begging her to leave in order to spare his reputation. Finally, Dyke eliminates everyone except Luka Patson. Hopelessly in debt, he had accepted bribes from Xavier to prevent Hawley from talking to Daniels about his ruby field. Daniels discovered this and Patson murdered him. When the train arrives in Bombay, Dyke turns him over to the authorities.