It is 1916 and the allies are losing the Great War. It had been a terrible two years; opening with that note of 'over by Christmas' optimism. Mycroft Holmes enlists the assistance of his brother Sherlock Holmes, 'You owe it to your country, ' says Mycroft who desperately needs Sherlock to investigate a female spy who is travelling too frequently between France and Germany.

Holmes and Watson take off for the Western Front disguised as army officers but whatever action they take, the spy always eludes them. Holmes and Watson are parachuted into enemy territory only to discover that they are almost too late. The plot involves a secret tunnel and, of course, Holmes and Watson satisfactorily fulfil their mission.