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. Wanting to know more, he took it to his English teacher Miss Isles, not realising the chain of events that he was setting in motion. Miss Isles 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 Isles disappeared on a class field trip, and Steven's memory won't allow him to remember what happened.

Now, out of prison after a long stretch, Steven decides to investigate the mystery that has haunted him for decades. Was Miss Isles 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 Isles, Steven revisits the people and places of his childhood. But it soon becomes clear that Edith Twyford wasn't just a writer of forgotten children's stories. The Twyford Code has great power, and he isn't the only one trying to solve it...