A dying informant directs Thomas Pitt towards an international plot, and the assassin fleeing the scene draws him across the Channel to Saint-Malo. While Pitt follows the trail abroad, Special Branch chief Victor Narraway is accused of embezzlement and removed from office.
Charlotte accompanies Narraway to Ireland to investigate the allegation and the enemies who arranged it. The two inquiries converge on a conspiracy capable of destroying careers and destabilising the British government, forcing the Pitts to work apart while confronting treachery within the institutions they serve.