Serena is a Cambridge mathematics graduate who vastly prefers literature over numbers. Pushed into her degree by her ambitious mother, she struggles academically but finds an escape in reading fiction voraciously. She accepts a low-ranking job at MI5, where she files intelligence reports while secretly seeking the validation she never received from her emotionally distant father. Her passion for books eventually lands her a spot on a covert literary operation.

Thomas is a small, pale academic and aspiring writer living on the coast. He writes short stories and articles criticizing oppressive regimes, making him an ideal candidate for MI5's covert funding operation known as Sweet Tooth. Though he is comfortable in the safe, predictable world of academia, he harbors a strong desire for literary success and craves genuine feedback on his creative work.

Tony is a 54-year-old historian and Cambridge university tutor. He presents himself as an erudite, sophisticated man of the world who deeply enjoys discussing politics, history, and international relations. He takes Serena to his isolated country cottage in Suffolk, directing her reading habits and grooming her for a future position within the British intelligence agency.

Max is a 32-year-old intelligence officer at MI5 who takes a keen interest in Soviet matters. He is an awkward, somewhat ill-dressed man who forms a tentative connection with Serena as they spend their spring lunch breaks talking on park benches. He views literature as largely frivolous, preferring strictly non-fiction, but assists in managing the Sweet Tooth operation.

Shirley is a working-class typist at MI5 who operates at twice the speed of her middle-class colleagues. She is well-traveled, sexually liberal, and openly dismissive of authority, making her a distinct outsider within the rigid intelligence agency. She becomes Serena's primary social companion, bringing her to loud, busy pubs in the evenings.

Serena's mother is a hardworking, self-sacrificing woman who actively helps her husband rise in the Church of England. Harboring a secret feminist streak, she channels her unfulfilled ambitions into her eldest daughter, insisting that Serena study mathematics at Cambridge despite the girl's obvious preference for literature.

The Bishop is a high-ranking clergyman in the Church of England and the father of Serena and Lucy. He is a profoundly emotionally distant man who views himself more as a title and a set of ceremonial robes than a parent, consistently withholding basic affection and validation from his daughters.

Lucy is Serena's younger sister. Lacking her sibling's strict academic discipline, she rejects conventional social expectations entirely. She abandons her medical degree, associates with counterculture figures, and lives a wayward, impulsive life that provides a sharp contrast to Serena's strict adherence to government rules.

Peter Nutting is an older, high-ranking official at MI5. He is the architect behind the Sweet Tooth operation, a covert scheme designed to fund right-leaning artists and writers to combat communist influence in culture. He evaluates Serena's literary knowledge to ensure she is the right person to approach potential candidates.

Harry Tapp is a stern, serious intelligence officer who conducts Serena's entry interview and later helps recruit her for the Sweet Tooth operation. He represents the heavily bureaucratic, formal nature of the British intelligence services during the 1970s.

Jeremy is a university student who dates Serena during her final academic stages. Their relationship is lackluster, and his primary impact on Serena's life is introducing her to his older history tutor, setting her life on a completely different trajectory.

Millie Trimingham is a single mother and a highly skilled intelligence officer who rises rapidly through the male-dominated ranks of MI5. She serves as a career template and a powerful aspirational figure for incoming female recruits like Serena.

Frieda is Tony Canning's wife. She prefers not to spend time at their rustic country cottage, inadvertently providing her husband with the perfect isolated location to conduct his secret affairs with younger women.

Luke is Lucy's long-haired Irish boyfriend. He actively participates in the counterculture lifestyle that Lucy has embraced and provides a stark political and cultural contrast to the conservative, state-aligned work Serena does.

Ruth is a doctor and Max Greatorex's fiancé. Her existence comes as a surprise to Serena, instantly complicating the developing workplace dynamic between the MI5 officers.