Christophor Morden lives by night. His day-brother, Alexsander, knows only the sun. They are two souls in a single body, in a world where identities change with the rising and setting of the sun.

Night-brother or day-sister, one never sees the light, the other knows nothing of the night. Early one evening, Christophor is roused by a call to the city prison. A prisoner has torn his eyes out and cannot say why. Yet worse: in the sockets that once held his eyes, teeth are growing.

The police suspect the supernatural, so Christophor, a member of the king's special inspectorate, is charged with finding the witch responsible. Night-by-night, Christophor's investigation leads him ever further from home, toward a backwards village on the far edge of the kingdom. But the closer he gets to the truth, the more his day-brother's actions frustrate him.

Who is Alexsander protecting? What does he not want Christophor to discover? And all the while, an ancient and apocalyptic ritual creeps closer to completion...