On Christmas Eve, DCI Marcus Priestley is aboard a Santa's Special at a heritage railway, with his wife Helen and their two children. The steam train is halted and a signalman is found dead. Helen, a doctor and psychiatrist, examines him and concludes he was murdered. Derbyshire police have the case taken off them by the Met, who rig an autopsy to conclude he died accidentally. Helen insists Marcus conduct his own unofficial investigation. He meets with the British Secret Intelligence Service and the CIA. When the suspect's phone is traced to Stockholm, he flies out with Lin Plummer, a DC. Ginny Long from the CIA meets them at the airport, and together the three set about tracking down the assassin, aided by Swedish Security.