Forty years ago, Steven Smith found a copy of a famous children's book by disgraced author Edith Twyford, full of strange markings and annotations.

Wanting to know more, he took it to his 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 a secret code that ran through all Twyford's novels. Then she 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? But as Steven revisits the people and places of his childhood, seeking answers, 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...