Commisar Ciaphas Cain - hero of the Imperium and renowned across the sector for his bravery and valour - is sent to help maintain order on an outpost world on the borders of Tau space. But when the alien ambassador is murdered and the situation quickly spirals out of control, Cain and his regiment of Valhallans find themselves in the middle of a war. As the Imperial Guard struggle to contain worldwide civil insurrection, can the wily Commisar Cain identify the real villian before the planet is lost to the Imperium forever ?