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.