England is being ruled by George III "the Mad King", and in London, at Mrs. Merlin's Academy for Select Young Ladies, one of Mrs. Merlin's beautiful young female operatives is being assigned a dangerous mission. Young Shannon, who had appeared at the end of The Spy Wore Silk to help her fellow-operative Siena achieve her mission, is now embarking on this first mission of her very own. England and Russia are in a fragile alliance, and France is trying to disrupt that alliance. France has sent its own lethal operative--an assassin named D'Etienne who already has murdered a Prussian envoy in Warsaw, and most recently has killed Russia's head of intelligence in Brussels. Now D'Etienne is in Ireland secretly teaching a band of Irish nationals how they could stage a revolt against England; and from there D'Etienne is scheduled to find and murder the Scottish ballistics expert Angus McAllister, whose death would be a terrible loss to Britain's army.

Shannon's assignment: to find and kill D'Etienne as soon as possible.