Assigned to the fractious Valhallan 296th/301st regiment, Commissar Ciaphas Cain has barely enough time to mold them into a semblance of a fighting unit, before it is deployed to Gravalax, which is threatening to secede to the Tau Empire. Once on the ground, the situation proves to be much more complicated than it appears, and Cain makes the surprise acquaintance of the Inquisitor sent to sort out the mess.