Summer, 1643. England is at war with itself. King Charles I has fled London, his negotiations with Parliament in tatters. For Thomas Hill, a man of letters quietly running a bookshop, knowledge of the war is limited to the rumours that reach the local inn. When a stranger knocks on his door one night, 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 there is evidence of a traitor in their midst. And when a vital message is intercepted, he must decipher it to reveal the king’s betrayer and prevent the violent death that failure will surely bring.