The year is 1972; the Cold War is far from over and Serena Frome; in her final year at Cambridge; is being groomed for MI5. Sent on Operation Sweet Tooth - a highly secret undercover mission - she meets 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 inventing whom? To answer these questions; Serena must abandon the first rule of espionage - trust no one.