According to legend, medieval heretics, the Cathars, owned a priceless treasure - their own Grail, for the possession of which the entire Roman Catholic Church was ready to go to great lengths. Papal spy Klaas Elke was tasked by Cardinal Treve to find the last surviving Perfect Cathar, Guillaume Belibaste, and hand him over to the Inquisition. However, it turned out that in fact the hunt is not so much for the heretic himself, but for the sacred treasure of which he is the keeper. Belibast was captured and, after torture, put on fire, and the treasure disappeared without a trace along with Klaas... Kasya Kuznetsova and her friend Kirill decided to spend a vacation near Belozersk. However, they were unable to rest: several murders occurred and Kasya, as usual, could not stay away. During the investigation, she has to visit Paris, finding herself in the epicenter of a centuries-old religious war for the possession of a sacred relic!..