Banished for the sin of mercy, Severian, one of the ancient guild of Torturers, flees from exile. In a mountain wilderness he meets the Alzabo, in whom those eaten seem to live on, adopts a son only to lose him in battle, discharges an old debt of vengeance, encounters fanged aliens who hide behind masks of beauty, and helps the people of the floating islands in their unending battle for freedom.

Now, alone again, Severian wanders in search of his destiny, guided by the glowing power of a hidden, awesome weapon.