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 has 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 crimson coloured, enamelled metal, in the shape of the letter Z. Temperley sets off in pursuit of the mysterious woman, only to find himself embroiled in a cross-country chase on the tail of a sinister serial killer.