Anna Zarides arrives in Constantinople in 1273 to prove the innocence of her twin brother Justinian, who has been exiled for conspiring to kill a nobleman. Disguising herself as a eunuch named Anastasius, Anna moves freely, using her skills as a physician to get close to key players in her brother's fate. Then she meets Giuiliano Dandolo, a ship's captain conflicted not only by his own mixed heritage, but by his growing feelings for Anastasius.