The series is a quest for the Holy Grail told as a murder mystery. It is set in the year 1933, in an alternate history Europe, where magic is real, feudalism persisted, and the Protestant Reformation was crushed by a still politically powerful Catholic Church. All of this is woven together as "... a meditation on the prophecies surrounding the advent of the Baháʼí era." The book takes its name from the Latin term meaning "King of the World". It is derived from the Cathar heresies of the Middle Ages, and taken up in works like "The Holy Blood and the Holy Grail". Within the Cathar context it seems to have been equated with the Demiurge.