In Byzantium, the capital of the 6th century Roman Empire, annual games are held to celebrate the founding of the city. Several courtiers, obliged by office to attend, idly watch the chariot races and the bear baiting from the imperial box. Suddenly they—and the crowd—are electrified as a magnificent bull surges into the arena. Those who worship Mithra make quiet reverence to the sacred animal while a trio of bull leapers enters in his wake. John, Lord Chamberlain to the Christian Emperor, is among those surprised into breathing tribute. He soon receives a further shock: surely the lovely young girl vaulting the beast had once been his lover.

Later, making his way home through the thronged streets, John stumbles over the body of his friend Leukos, Keeper of the Plate. There are plenty of witnesses: an Egyptian brothel keeper, a young mason working on the Church of the Holy Wisdom, a mad stylite, a henpecked innkeeper, and the bull leapers from Crete. Now duty and guilt demand that John discover the murderer....