On the ice world of Simia Orichalcae a spate of mysterious disappearances is causing unrest amongst the mine-workers, and, as senior officer of the Astra Militarum, Commissar Ciaphas Cain is nominated to investigate. Unbeknownst to him, the planet is right in the path of a major ork incursion and, as the savage greenskins attack, a malevolent evil begins to stir deep in the ice caves.