Agatha Christie in London to visit her agent boards a train preoccupied in the knowledge that her husband Archie is having an affair. She feels a touch on her back loses her balance but is pulled to safety from the rush of the incoming train. So begins a terrifying sequence of events. Her rescuer is no guardian angel but a blackmailer and Agatha must use every ounce of her cleverness to thwart an adversary determined to exploit her genius for murder to kill on his behalf.