England is at war with itself. King Charles I has fled London, his negotiations with Parliament in tatters. The country is consumed by bloodshed.
For Thomas Hill , a man of letters quietly running a bookshop in the rural town of Romsey, knowledge of the war is limited to the rumours that reach the local inn.
When a stranger knocks on his door one night and informs him that the king's cryptographer has died , everything changes. Aware of Thomas's background as a mathematician and his expertise in codes and ciphers , the king has summoned him to his court in Oxford .
On arrival, Thomas soon discovers that nothing at court is straightforward. There is evidence of a traitor in their midst. Brutal murder follows brutal murder. And when a vital message encrypted with a notoriously unbreakable code is intercepted, he must decipher it to reveal the king's betrayer and prevent the violent death that failure will surely bring.