Spring, 1897. London. Margaret Harkness, now in her early forties, must leave England for her health but lacks the funds. A letter arrives from her old friend Professor Bell, her old comrade in the hunt for Jack the Ripper and the real-life inspiration for Sherlock Homes. Bell invites her to join him in Germany on a mysterious mission for the German government involving the loss of state secrets to Anarchists. The resolution of this commission leads to her being stalked through the streets of London by a vengeful man armed with a powerful and nearly silent air rifle who has both Margaret and Queen Victoria in his sights. Margaret finds allies in Inspector James Ethington of Scotland Yard and his fifteen-year-old daughter, Elizabeth, who aspires to follow in Margaret's cross-dressing footsteps.

The hunt is on, but who is the hunter, and who the hunted as the day approaches for the Queen's Diamond Jubilee when the aged empress will sit in her open carriage at the steps of St Paul's Cathedral? The entire British Empire holds its breath as the assassin, Margaret, and the Queen herself play for the highest of stakes with the Queen's Gambit.