Serena Frome, the beautiful mathematician daughter of an Anglican bishop, has a brief affair with an older man during her final year at Cambridge, and finds herself being groomed for the Intelligence Service. The year is 1972: Britain, confronting economic disaster, is being torn apart by industrial unrest and Irish terrorism and faces its fifth state of emergency. The Cold War has entered a moribund phase but the fight goes on and MI5 hesitates at little to influence hearts and minds. Serena, a compulsive reader of novels, is sent by her new employers on a secret mission that brings her into the literary world of Tom Haley, a promising young writer. First, she loves his stories, then she begins to love the man. Can she maintain the fiction of her undercover life? And who is deceiving whom? To answer these questions, Serena must abandon the first rule of espionage - trust no one.