Serena Frome wants to study English at university however she bows to parental pressure and goes to Cambridge to study mathematics. In her final year she has a brief affair with an older man who encourages her to apply for a job at MI5. Ian McEwan has set the book in the early 1970s at the end of the Cold War. It is also the time of IRA bombings, petrol shortages, an economic crisis and industrial unrest in Britain. Serena, who has maintained her interest in reading, particularly modern literature, is seconded to the project, Sweet Tooth, where her mission is to encourage a young novelist to accept a grant from a literary agency funded by MI5. It is believed that the writings of the targeted novelist along with other authors in the program will promote the MI5 message.
Books and writing are a major thread of the book. It is the short stories of Tom Haley that have brought him to the attention of MI5 and some of these are included throughout the book along with literary discussion between Tom and Serena. Serena soon discovers that working undercover is not always easy, especially as her relationship with Tom develops. Should she tell him about her real role and risk losing her job and his love or should she continue with the situation as it it in the hope that he will not discover the truth. The book, although set within a spy agency, is not a thriller but is about truth, trust and betrayal. It also investigates the relationship between authors and readers.