Arthur should be happier than ever. Professor Sherlock Holmes has accepted him into the esteemed Circle of Light, and his friendships are stronger than ever. However, Dinah Grey's ominous letter and her dangerous hold over his little sister, Mary, have left Arthur riddled with anxiety. After the annual first-year retreat to the storied Lake District is announced, another letter arrives and makes the key to Mary's salvation clear: Arthur needs to insert himself in Sherlock's upcoming investigation...as Dinah's spy. While sticking close to Holmes on the trip, Arthur meets George Rankin, one in a long line of protectors of the Brazen Head, a powerful relic whose wearer can ask any question and have it answered. But there are more eyes on Sherlock and the relic than just Grey's. Arthur will have to walk the thin line between truth and deception in order to save his sister, but it might cost him more than he realizes.