Phaedrus, an enslaved gladiator in northern Britain in the first century, earns his freedom by killing his best friend, a fellow gladiator, in a final fight to the death. Within days of leaving the arena he is recruited by leaders of a tribe from the far north to impersonate Prince Midir, who has been robbed of his right to kingship in a brutal attack by followers of the current ruler, Queen Liadhan. As Midir, Phaedrus is charged with reestablishing his kingship and the tribe's rule in the land. In this world of superstition and ancient ritual, of fierce loyalties and intertribal rivalry, Phaedrus finds companionship and love, and something more - a purpose and a meaning for his life as he comes to fully understand the significance of the Mark of the Horse Lord.