Rolling back the clock from the modern date of 1938, Inspector Blair relives his earlier days in service during the Great War. It’s 1914, and he is on the guard against potential spies. A suspicious letter comes into his possession, the blank sheet of paper inside is submitted to a couple developing tests, and plans of a railroad bridge come into view.
Blair springs into action to learn where that bridge is located. His friend Paddy helps identify the bridge, and using train schedules, he tries to deduce a likely time for trouble. At the bridge, Blair finds the bridge and tracks in good shape, but remains alert for trouble.
Paddy is the train expert to point out what to expect, and what might go wrong. Suddenly a saboteur does his work, and seems to get away. With the train racing for disaster there’s not time to chase after the spy. Can the train be stopped in time?
Thanks to Blair’s suspicions, and Paddy’s knowledge of the railroad the day is saved for the troop train.