Setting out to exterminate the upstart Tau Empire before they become the threat, the Ultramarines under Captain Atheus discover that the xenos may be more of a menace than they originally believed. The Imperium of Man takes its bloody revenge upon the expansionist tau in a war of dizzying spectacle. For the first time, the daredevil warriors of the Ultramarines Assault Company go to war en masse, fighting in the skies, in the streets, and even in the prototype testing facilities of the tau Earth caste. However, Sergeants Sicarius and Numitor must overcome their hunger for glory as the brightest stars of the Tau Empire, Commanders Farsight and Shadowsun, hunt them to the brink of disaster. Tempers run short as battle-brothers fall, ammunition runs out and the course of the war takes ever-darker twists and turns. With two warrior cultures struggling for a vital edge and the body count spiralling towards a terrible conclusion, can notions of honour and duty survive at all?.