Aventurine Morrow is alone in Lincoln, England, attempting to overcome writer's block while researching a new subject, Katherine Swynford. Here she befriends a cathedral tour guide, Henry Hallsey, whose daughter Nicola has gone missing. When a woman is found dead on the grounds of the Old Bishop's Palace, and the Swynford Jewel is stolen, Aventurine falls under suspicion.

With police dogging her every step, Avi must seek the help of wily former spy, Genevieve Smithson. In the midst of all this, Gio Constantine reappears-but Gio might not merely be the sensitive singer-songwriter he appears to be.