Marneus Augustus Calgar is the Lord of Ultramar, one of the greatest realms of the Imperium. As Chapter Master of the Ultramarines, he leads one of the most prestigious and powerful Space Marine Chapters devoted to the defence of Mankind. In him, the blood of the Primarch Guilliman runs still. When Calgar receives a summons to the world of Zalidar, he is unaware, as are the world's inhabitants, that it is under attack by the Ork Warlord Brug. When his vessel is shot down, Calgar and his honour guard must trek across Zalidar's jungle through ork-held territory to reach the city of Zalathras. Here a terrible siege ensues against a vast greenskin horde. Only Calgar can hope to prevail against such odds in a brutal conflict that will give rise to a legend...