Forty years ago, Steven Smith found a copy of a famous children's book by disgraced author Edith Twyford, its margins full of strange markings and annotations. Severely dyslexic and wanting to know more, he took it to his remedial English teacher Miss Iles, not realising the chain of events that he was setting in motion. Miss Iles became convinced that the book was the key to solving a puzzle, and that a message in secret code ran through all Twyford's novels. Then Miss Iles disappeared on a class field trip, and Steven has no memory of what happened to her. Now, out of prison after a long stretch, Steven decides to investigate the mystery that has haunted him for decades. Was Miss Iles murdered? Was she deluded? Or was she right about the code? And is it still in use today? Desperate to recover his memories and find out what really happened to Miss Iles, Steven revisits the people and places of his childhood. And as he does so, he records the story of his life in the form of voicemails and voice memos for his estranged and long unknown son, a professor of mathematics. But it soon becomes clear that Edith Twyford wasn't just a writer of forgotten children's stories. The Twyford Code is valuable, and he isn't the only one trying to solve it.