Sir Wilfred Saxonby is travelling alone on the 5 o'clock train from Cannon Street, in a locked compartment. The train slows and stops inside a tunnel; and by the time it emerges again, Sir Wilfred has been shot dead, his heart pierced by a single bullet. Suicide seems to be the answer, even though no motive can be found. Inspector Arnold of Scotland Yard thinks again when he learns that a mysterious red light in the tunnel caused the train to slow down, and it soon becomes apparent that the dead man fell victim to a complex conspiracy.