As the snow falls thickly on Newcastle, the shouts and laughter of Christmas revellers break the muffled silence. Detective Joe Ashworth and his daughter Jessie are swept along in the jostling crowd onto the Metro. But when the train is stopped due to the bad weather, and the other passengers fade into the swirling snow, Jessie notices that an old lady hasn't left the train: Margaret Krukowski has been fatally stabbed as she sat on the crowded train. Nobody, including the policeman himself, sees the stabbing take place. Margaret's murderer is seemingly invisible; her killing motiveless. Why would anyone want to harm this reserved, elegant lady?