Agatha Christie, in London to visit her literary agent, boards a train, preoccupied and flustered in the knowledge that her husband Archie is having an affair. She feels a light touch on her back, causing her to lose her balance, then a sense of someone pulling her to safety from the rush of the incoming train. So begins a terrifying sequence of events. Her rescuer is no guardian angel; rather, he is a blackmailer of the most insidious, manipulative kind. Agatha must use every ounce of her cleverness and resourcefulness to thwart an adversary determined to exploit her genius for murder to kill on his behalf.