Constantinople, May, 1453. In the dying days of the Byzantine Empire, as the city prepares for a final onslaught by the Ottoman Turks, Isaak Metochites and his family are entrusted with a silver reliquary carved with the figure of a weeping angel and the inscription: "Behold the Proof of Chora, Container of the Uncontainahle". Four hundred years later, magistrate Kamil Pasha is plagued by thefts of antiquities from mosques and churches and a series of murders in which the bodies bear the same distinctive mark. Sources lead Kamil to a hidden sect descended from Abyssinian slaves living in an abandoned cistern in Istanbul"s gritty underworld. The re-emergence of the forgotten reliquary sets off a brutal race between those sworn to protect it and those who will stop at nothing to gain its explosive secret.