Inspector Dyke (Edmund Lowe), of His Majesty's Service in India, is assigned the task of unraveling the circumstances , aboard the Bombay Mail express train from Calcutta to Bombay, surrounding the mysterious death of one Sir Anthony Daniels (Ferdinand Gottschalk). He treats the case as murder and seeks to find out why it was done and who did it. Suspicion falls on a large number of passengers, including the strange Sonia Smeganoff (Shirley Grey), who isn't all she appears to be ; the Maharajah of Zungore (Walter Armitage) and a character known only as Xavier (John Davidson). Dyke is careful where he steps as a cobra is also on the train.