With French Revolutionary agents travelling freely but secretly between Ireland and England and war with France inevitable, the English government of Mr Pitt is desperately anxious to uncover any plots between Irish rebels and the French.
Lieutenant James Merriman with his ship, the sloop Aphrodite, is ordered to the Irish Sea to assist the principal Treasury agent Mr. Grahame in this work. Merriman is plunged headlong into the world of espionage and when Grahame is seriously wounded it falls to Merriman to carry on the investigation.
Young James Merriman must keep all his wits about him to foil these plans especially when his adversary is revealed to be an exceptional French agent Henri Moreau who hopes that by helping the Irish to throw off the English yoke, France will be able to use Irish ports from which to attack England.