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.
Temperley has a brief encounter with a beautiful young woman, but she flees the scene.
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 the mysterious woman from the hotel, and finds himself embroiled in a cross-country chase—by train and taxi—on the tail of a sinister serial killer.
