It is December 1657. John Grey has returned to the study of law in London. He receives a mysterious invitation which seems to have come from some of his mother's royalist friends - possibly members of the largely ineffectual secret organisation, the Sealed Knot. He is amused rather than otherwise that they would think him stupid enough to join them and thinks it may be entertaining to tell them so to their faces. On arriving at Sir Richard Willys's chambers at Gray's Inn, however, he finds that the invitation was never intended for him - to his horror, Grey is warmly greeted as a royalist conspirator sent over from Brussels to murder Oliver Cromwell. It soon becomes clear that the man he has been mistaken for is his own father, a long-time royalist exile. The question is how he can prevent the murder without betraying his father.