Commissar Ciaphas Cain is a renowned and revered hero of the Imperium, a man who has faced and survived some of the vilest creatures the universe can throw at him. But when he is sent to a river-world, he must deal with a dangerous enemy, an enemy whose true identity remains unknown. As his vessel traverses the straits of the planet, Cain must uncover the face of this new foe so that he can understand and escape it. Caught in the enemy crossfire, the commissar has no place to run, and his nerve will be tested to the very limits.