On a dark November evening, Sir Wilfred Saxonby is travelling alone in 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 minutes later, Sir Wilfred has been shot dead, his heart pierced by a single bullet.
Suicide seems to be the verdict, even though no motive can be found, but Inspector Arnold of Scotland Yard thinks again when he learns that a mysterious red light in the tunnel caused the train to slow down.
Can there be a connection with Sir Wilfred's seemingly troubled family life, his highly successful business, or his high-handed and unforgiving personality? And what is the significance of the wallet found on the corpse, and the bank notes that it contained?