Knight Tem­plar Jonathan St. Clair is gar­risoned in the city of Acre in 1290s, the twi­light of the Cru­sades. In pos­ses­sion of a scroll with a cryp­tic Kab­bal­is­tic inscrip­tion, St. Clair becomes more inter­est­ed in learn­ing Kab­bal­ah than fight­ing Mus­lims when he learns that the inscrip­tion holds the key to unlock the secrets of Jerusalem’s Tem­ple Mount. But no Jew­ish schol­ar in Acre is able to deci­pher the inscrip­tion, and time is run­ning out; Acre will soon come under siege, and the one man able to divine the scroll’s mean­ing, Rab­bi Samuel, the last Gaon of Bagh­dad, is tar­get­ed for assas­si­na­tion by a rival rab­bi because of their sharp dis­agree­ments in the Mai­monidean Con­tro­ver­sy. St. Clair appren­tices him­self to Rab­bi Samuel and togeth­er with Zahi­ra, a beau­ti­ful Mus­lim woman who had been avoid­ing a local emir by hid­ing in a lep­er colony, and with the assis­tance of al-Hasani, a bril­liant Mus­lim physi­cian and car­tog­ra­ph­er, the knight and rab­bi trav­el to Jerusalem, risk­ing every­thing to ful­fill their shared and sacred des­tiny as guardians of the Tem­ple Mount.