It is 1761. The British prime minister, William Pitt, is faced with the need to relieve French pressure on Hanover. He is against sending more British regiments to the continent and instead decides to draw the French army away from Germany by a repeat of the descents on the French coast that he tried three years earlier. The chosen target is Belle Isle, an important island that lies between the principal French Atlantic naval ports.

George Holbrooke's ship Argonaut is sent ahead of Commodore Keppel's squadron to gather intelligence on the French army's movements by inserting an intelligence agent into Brittany. The agent is betrayed and wounded, and his contact in France must be rescued from the certainty of a traitor's death. Holbrooke finds that the only way to accomplish his orders is to land on French soil himself, by moonlight, and seek out the agent's contact. In a ruined cottage close to the sea, he makes a surprising discovery.

Treacherous Moon is the twelfth Carlisle and Holbrooke novel. The series follows the two sea officers through the Seven Years War and into the period of turbulent relations between Britain and her American colonies prior to their bid for independence.