In the summer of 1707, two adventurers on the roads of Normandy come upon a brutal ambush. Saving a desperate man from the king's guards, they accept a dying stranger's final plea: to deliver a secret letter to a nameless prisoner held in the fortress abbey of Mont St Michel.

With gold and a ring as payment, the pair sets off-only to find themselves hunted by royal agents and entangled in a conspiracy that reaches into the highest echelons of power. A chance encounter with a dissolute nobleman, who is their near double in looks, opens an unexpected path, and they swap identities to carry out a dangerous mission.

Now, one must ride toward the towering island monastery, bearing letters that could mean life or death. With spies lurking in every village and the tides of the sea cutting off escape, failure is not an option. But what secret does the prisoner keep-and what price will the rescuers pay to uncover it?