Warriors of several Space Marine Chapters are preparing to defend the Cardinal World of Almas from the invasion of unholy traitors from the Word Bearers Legion. However, the Imperials do not know that the goal of their enemy is not the conquest of another planet... Primaris Lieutenant Heid Calder, a skilled warrior-diplomat, on the orders of Roboute Guilliman, is sent to Almas, a remote possession of the Ecclesiarchy. He must save this world at any cost. As a hellish armada of chaos-worshippers breaks through the system's borders, Calder discovers that the planet's Cardinal Governor - a charismatic and mysterious man in whose hands all spiritual and temporal power is concentrated - is hiding something. When it turns out that the arch-traitors need not only the imperial world itself, the housecarl of the sons of Dorn decides to reveal the secret carefully kept on Almas. The Word Bearers' invasion engulfs the entire system in the flames of battle, and to achieve victory, the forces of the Ecclesiarchy and the Adeptus Astartes - so different in matters of faith, strategy and politics - will have to fight side by side.