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 & stops inside a tunnel; & 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 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 Finding himself stumped by the puzzle Arnold consults his friend Desmond Merrion a wealthy amateur expert in criminology Merrion quickly comes up with an 'essential brainwave' & helps to establish how Sir Wilfred met his end but although it seems that the dead man fell victim to a complex conspiracy the investigators are puzzled about the conspirators' motives as well as their identities Can there be a connection with Sir Wilfred's seemingly untroubled family life his highly successful business or his high-handed & unforgiving personality? & what is the significance of the wallet found on the corpse & the bank notes that it contained?