Spring, 1661. After years of civil war followed by Cromwell's joyless rule, England awaits the coronation of King Charles II. But when two respectable gentlemen are found in a foul lane with their throats cut, it becomes apparent that England's enemies are using the newly re-established post office for their own ends. There are traitors at work and plans to overthrow the king. Thomas Hill, in London visiting friends, is approached by the king's security advisor and asked to take charge of deciphering coded letters intercepted by the post office. He finds himself dragged into the hunt for the traitors and the murderer, but will he find them before it's too late?