Oxford, England. After a decade abroad, Martha Thornhill has returned home to the city whose ancient institutions have long defined her family. But the ghosts she had thought to be at rest seem to have been waiting for her to return. When an anonymous letter is delivered to the Clarendon English Dictionary, where Martha is a newly hired senior editor, it's rapidly clear that this is not the usual lexicographical enquiry. Instead, the coded letter hints at secrets and lies linked to a particular year.

The date can mean only one thing: the summer Martha's brilliant older sister Charlie went missing.

When more letters arrive, Martha and her team pull apart the complex clues within them, and soon, the mystery becomes ever more insistent and troubling. Because it seems Charlie had been keeping a powerful secret, and someone may be trying to lead the lexicographers towards the truth that will unravel the mystery of her disappearance. But other forces are no less desperate to keep their secrets well and truly buried, and Martha and her team must crack the codes before it's too late.

Audiobook includes an exclusive introduction by Susie Dent and Q&A between Susie and Jack Edwards from YouTube.