Cambridge student Serena Frome's beauty and intelligence make her the ideal recruit for MI5. The year is 1972. The Cold War is far from over. England's legendary intelligence agency is determined to manipulate the cultural conversation by funding writers whose politics align with those of the government.

The operation is code named "Sweet Tooth." Serena, a compulsive reader of novels, is the perfect candidate to infiltrate the literary circle of a promising young writer named Tom Haley. At first, she loves the stories. Then she begins to love the man. How long can she conceal her undercover life? To answer that question, Serena must abandon the first rule of espionage: trust no one.