In London, Lionel's family are unaware of his past role in The Security Services. Now Alzheimer's disease makes him dependent on others for almost everything. The light in his darkness is the special relationship with his granddaughter Emily, whose father recently died saving Lionel from an accident.
In Hastings, Jeremy Catchpole, Worm to his friends, is a larger than life second hand bookseller. An enthusiastic participant in the many bizarre local festivals and a keen amateur sleuth.
As Lionel's ex-colleagues become increasingly concerned about keeping the secret of his most important mission, Worm is shown two story books previously owned by the old man. Although both are copies of the same children's book, one has been tampered with in an intriguing way. A painting and an engraved pocket watch also seem to hold clues to Lionel's past.
Suddenly their two worlds collide as Worm investigates and realises that he and the people around him are all in grave danger.