In August 1643, scholarly bookseller Thomas Hill is summoned by the King to his court in Oxford to take over as chief cryptographer. As an opponent of the war, he goes reluctantly.

Once there, he finds his old university town riven by extremes of poverty and excess and discovers that his pedecessor was murdered and that there may be a spy at court. When a message, encrypted with the ‘unbreakable’ Vigenère cipher, is intercepted, Thomas must break it to reveal the identity of the traitor.

He witnesses a bloody and pointless battle at Newbury, is thrown into the notorious Oxford Castle gaol, and survives attempts on his life, before finally proving who the traitor is and thereby foiling a parliamentary plot which would have changed the course of the war.