Julian Lawndsley has left a career in the financial sector in London to open a bookstore in a small seaside town in East Anglia. Soon after opening the shop, a Polish immigrant named Edward Avon comes in, not to buy books but to chat. He urges Lawndsley to open a section of the store in the basement called the Republic of Literature, which would offer only the classics.

Edward Avon is later revealed to be a retired agent for MI6 and a former communist. Avon claims to have known Lawndsley's father at school before he joined a cult. Lawndsley's father had a prolific sex life and ran up debts. As he continues to learn more about Avon's background, Lawndsley is fascinated by his different identities, and he wonders which are performances and which are real. He soon becomes entwined with the life of Avon's family: his wife Deborah – also a former top British intelligence agent – who is terminally ill, and his daughter Lily, a single mother.

In another narrative strand, Avon is being investigated by the secret service's head of domestic security, Stewart Proctor. Proctor is suspicious of anyone like Avon, an ex-Communist, who demonstrates an absolute commitment to anything and who is thus a grave security threat. Proctor goes looking for Avon, suspecting him of being the source of an intelligence leak, and drags Lawndsley into his mission in the process.