The year is 1972. Britain is confronting economic disaster. The cultural cold war continues, the country is being torn apart by industrial unrest and terrorism, and faces its fifth state of emergency. Serena Frome, the beautiful daughter of an Anglican bishop, has a brief affair with an older man during her final year as a student at Cambridge, and finds herself being groomed for the intelligence services. Recruited into MI5 against a backdrop of the Cold War, Serena, a compulsive reader, is assigned to infiltrate the literary circle of a promising young writer. England's legendary intelligence agency is determined to manipulate the cultural conversation by fundng writers whose politics align with those of the government. The operation is code named "sweet tooth". This 'secret mission' brings her into the literary world of writer Tom Haley. 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.