Richard Temperley arrives at Euston station early on a fogbound London morning. He takes refuge in a nearby hotel, along with a disagreeable fellow passenger, who had snored his way through the train journey. But within minutes the other man has snored for the last time - he has been shot dead while sleeping in an armchair. When the police arrive, Detective Inspector James discovers a token at the crime scene: 'a small piece of enamelled metal. Its colour was crimson, and it was in the shape of the letter Z.' Temperley sets off in pursuit of a mysterious woman from the hotel and finds himself embroiled in a cross-country chase on the tail of a sinister serial killer.