The game’s plot line involves the Forge World Graia (a planet devoted to military production) that has been invaded by alien Orks. The game follows a Command Squad of three of the Ultramarines, 2nd Company Commander and playable character Captain Titus, his immediate subordinate Veteran Sergeant Sidonus, and a relatively inexperienced and by-the-book Tactical Marine named Leandros.
Initially upon arrival the squad provide assistance to the outnumbered and overrun Imperial Guard regiments fighting on the planet’s surface, which after the loss in battle of all higher-ranking officers are now being led by the sole surviving junior officer, Second Lieutenant Miranda ‘Mira’ Nero; she informs Captain Titus of the present situation and briefly accompanies the squad to their next waypoint. Titus agrees to silence the captured planetary defense cannon which has been preventing supply ships from reaching the beleaguered defenders. This done, Mira thanks the Space Marines and continues to help Titus however she can over the course of the game, amidst re-organizing the remaining Imperial Guard forces and attempting to hold the majority of the invading hordes at bay until the delayed reinforcement fleet arrives. The Space Marines then move on to continue with their own mission and, amidst securing the Titan Invictus (one of a class of giant, bipedal war machines that are produced primarily on Graia and the principal reason for the Ultramarines’ deployment there), answer a distress call from the injured Imperial Inquisitor Drogan, from whom they learn of a weapon that can wipe out the Orks, but which needs a power source that is located in a reactor below the Manufactorum. Titus retrieves it and activates the Psychic Scourge, but the Orks are not killed. Instead a Warp gate to the psychic realm of Chaos is opened, unleashing several daemons that massacre the surrounding Orks. Moments later, the Chaos Lord Nemeroth emerges from the portal. Using his Warp powers, he subdues the Space Marine squad as Inquisitor Drogan arrives via lift and approaches Nemeroth, revealing his allegiance to the Chaos Lord. It is then revealed that Drogan was killed at some point prior to meeting Titus and his corpse was possessed and used by Nemeroth as a sleeper agent. Titus gathers enough strength to grab the Power Source, which had been ejected from the Scourge, and Nemeroth is ambushed by Grimskull, the Ork Warboss in charge of their invasion of Graia, allowing the Space Marines to escape.