It is December 1657. John Grey, at his cramped desk in Lincoln's Inn, is attempting to resume his legal career. A mysterious message from a 'Mr SK' tempts him out into the snowy streets of London and to what he believes will be a harmless diversion from his studies.

Mr SK's letter proves to have been intended for somebody else entirely and Grey finds himself unwittingly in the middle of a plot to assassinate the Lord Protector - a plot about which he now knows more than it is safe to know. Can he both prevent the murder and (of greater immediate relevance) save his own skin? Both the Sealed Knot and Cromwell's Secretary of State, John Thurloe believe he is on their side, but he is unsure that either is on his. As somebody is kind enough to point out to him: 'You are a brave man, Grey. The life of a double agent can be exciting but very short.'

Grey just has to hope that prediction is wrong.