Graia is an Imperial Adeptus Mechanicus Forge World in the Segmentum Tempestus of the Milky Way Galaxy. The planet is home to the Legio Astraman, a Loyalist Titan Legion. Graia was the site of an invasion by over 1 million Orks led by the Warboss Grimskull in the late 41st Millennium in an attempt to seize the Warlord-class Titans being constructed within the planet's famous Ajakis Manufactorum. The invasion was stopped in its tracks by the intervention of the 203rd Cadian Shock Troopers and the 2nd Company of the Ultramarines led by Captain Titus, and the world was ultimately reclaimed for the Imperium by a full liberation fleet. During the battle Titus discovered that the Ork invasion had been a cover for the Chaos Sorcerer Nemeroth and his warband of Chaos Space Marines to seize experimental Imperial Warp technology that he intended to use to become a Daemon Prince and transform Graia into a new Daemon World under his rule. Titus successfully slew the sorcerer but came himself under suspicion of heresy because of his unusual resistance to Chaos sorcery.
A cogitator (computer) screen of the Adeptus Administratum provides an overview of the Imperial Forge World of Graia in the Segmentum Tempestus, one of the planets controlled by the Adeptus Mechanicus that specialise in the production of weapons and tools of war for the Imperial armies. Graia is also home to the Titan Legion known as the Legio Astraman ("The Morning Stars") and to the extremely rare facilities of the Ajakis Manufactorum that is capable of constructing and repairing Warlord-class Battle Titans. This capability leads the Administratum to grade the world as one of the Imperium of Man's most important strategic locations. After considering and rejecting a number of options, including an Exterminatus, the cogitator recommends combating an invasion of Graia by an Ork WAAAGH! 1 million Greenskins strong by means of a full-scale liberation fleet of the Imperial Navy carrying multiple regiments of the Imperial Guard. However, its analysis indicates that the Orks will overwhelm the world and take control of the Titan manufactorum facilities before that fleet can reach Graia through the Warp. The cogitator then recommends a delaying action be commenced in order to buy the necessary time for the fleet to be mobilised and arrive. Thus, the Administratum contacts the Ultramarines Chapter the Space Marines to carry out this delaying action, as Graia is an extremely valuable and important asset for the Imperium, one which the Greenskins cannot be allowed to have. The Ultramarines agree to deploy their entire 2nd Company to deal with the situation.
Upon his arrival in orbit of Graia leading the 2nd Company of the Ultramarines on an Ultramarines Strike Cruiser Fury of Descent, Captain Titus and two members of his company's Command Squad, his old friend Veteran Sergeant Sidonus and the young Battle-Brother Leandros, travel in a Thunderhawk gunship down into the atmosphere of the Forge World to survey disposition of the attacking Ork forces. The antiship guns on the Ork flagship, a Kill Kroozer, are rapidly destroying the Escort vessels that have been deployed into the Graian sky to contest the Greenskins' landing. Titus realizes that the most effective way to stop the Ork vessel's guns is to launch a quick boarding action against their flagship using Assault Marine Jump Packs, and destroy the ship's weapons directly. Titus proceeds with this plan, in spite of the displeasure expressed by Leandros, who points out that such a bold action is not the correct course of action for such a situation as described by the Codex Astartes since it may lead to the attacking force being scattered. Titus ignores Leandros' griping and orders his Thunderhawk's pilots to return to the Strike Cruiser after they begin the assault. Titus challenges the young Astartes to keep up with him before leaping out of the Thunderhawk and using his Jump Pack to soar through the contending vessels and land on the gun deck of the Ork flagship.
Brutally slaughtering a horde of Greenskins after landing on their vessel, Titus soon encounters the Ork Warboss Grimskull who is leading the WAAAGH! but is unable to ensure his destruction, though he does manage to turn the vessel's antiship guns on itself. Unfortunately, this action also damages the Ork vessel's propulsion and power systems and causes it to crash into the surface of Graia. Titus survives the crash, shakes free of the wreckage and heads out into the ruins of the planet's surface in the midst of a major manufacturing district. Titus soon meets up with both Leandros and Sergeant Sidonus who also survived the crash of the Ork Kroozer through the Orks fire had scattered them from landing with their captain on the ship. Together, the three Astartes proceed through Ork-held territory in the shattered ruins of what was once the Forge World's Industrial Zone 82 until they link up with the only surviving Imperial Guard officer on the planet, 2nd Lieutenant Mira of the 203rd Cadian Shock Troops Regiment. There they are told of the situation, in which the remaining Imperial Guard troops waiting in orbit are unable to move any men or equipment down to the planet's surface due to the Orks' capture of a massive kinetic planetary defence gun emplacement. This emplacement, Planetary Defence Battery Goliath, has been turned by the Greenskins to their own purposes and prevents the Imperial forces from making any further landings unless they want to see their transport ships being blown out of the sky. Titus realises that the Ork leader is more tactically-minded than most of his breed. Titus promises Mira that the Astartes will deal with the problem while she remains focused on strengthening the Imperial Guard outpost's defences.
Titus and his companions proceed once more into Ork territory, killing many of the Greenskins on their way to destroying the gun emplacement. They penetrate the inner workings of the domed Goliath battery through the debris field created by the arrival of an Ork Rok. The Astartes successfully destroy the battery by placing a Melta-Bomb on one of the defence battery's monstrous artillery shells. When the Orks fired this shell from the battery, it caused the battery's cannon to explode crippling it. After the battery's successful destruction, Lieutenant Mira's Imperial Guard support was able to begin landing on the world in their Valkyrie gunships and reinforce the Imperial defenders' beleaguered positions.
The three Space Marines then made their way towards the western outskirts of the Ajakis Manufactorum in order to secure any Battle Titans that the Orks were hoping to seize. To reach the manufactorum the Ultramarines first had to defeat a hovering anti-gravitic Ork battering ram using a Plasma Cannon emplacement that was located aboard an Adeptus Mechanicus cargo railway system.
Within the manufactorum the Astartes encountered a Servo-skull attempting to deliver a message. Once identifying Captain Titus, it plays a message left by Inquisitor Drogan, a member of the Inquisition's Ordo Xenos who was on the planet performing research when the Orks assaulted it. He informs the captain that the enemy is close to obtaining control of a Warlord-class Battle Titan being assembled on the Forge World, and that they must proceed to Graia's Ajakis Manufactorum to ensure the safety of the Titan. Titus then leads his men to the manufactorum, and are assisted by the Imperial Guard troops led by Corporal Antioch in gaining entry into the vast factory complex.
Once inside the manufactorum, the three Astartes seek out Drogan, who informs them that the Orks cannot have access to the power supply of the manufactorum which is a potent technological wonder of his own creation. Drogan leads the Space Marines through the massive facility, seeking out his unusual power source. Upon reaching the destination, Titus retrieves the object, which is a small cylinder wreathed in potent blue energies, which results in a deadly explosion due to the technology's inherent instability. Titus, however, manages to survive the blast, despite a message from Sidonus that the Inquisitor was sure of his death.
Titus then proceeds through the sewers of the manufactorum complex on Graia, eventually linking up with both of his fellow Astartes and the three continue on to the meet point with Drogan. Upon reaching him, the Inquisitor informs Titus that he shouldn't be alive, and that the energy source can create a direct link to the Warp and the potent energies of Chaos that lie within that parallel dimension. Titus and his men continue despite the Inquisitor's warnings, and eventually meeting up with Lt. Mira proceed to assist her in shoring up several key locations that the Imperial Guardmen are desperately trying to defend.
Drogan informs Titus that he has access to a weapon called the Psychic Scourge that could be used to completely wipe out the Orks by using the power of the Warp to overwhelm their natural gestalt psychic field of WAAAGH! energy and literally blast apart the Greenskins' minds all at the same time. Titus agrees that despite the risk of tearing the planet apart if the weapon is used, they must make use of it before the world is overrun. Securing Imperial transports, they proceed onward to Drogan's laboratory, where the weapon is located. After getting past the laboratory's automated security, Drogan tells Titus that the power source has to be inserted into the weapon's core, and that the Orks managed to cut off the power to his weapon. After solving both issues, Titus and his men watch as Drogan fires the weapon towards an Orbital Spire of one of the world's hive cities that is teeming with Orks, but the resulting blast instead rips open portals into the Warp that open all across the Forge World.
Titus and his men then prepare for the final showdown with Grimskull, but are instead interrupted by the arrival of scores of Chaos daemons and eventually a mighty Chaos Sorcerer named Nemeroth wearing a powerful suit of Terminator Armour. Nemeroth is accompanied by the warband of Chaos Space Marines that he leads who are known as the Chosen of Nemeroth. Nemeroth uses his sorcerous power to subdue both the Greenskin Warboss and the Ultramarines, and Titus can only watch as Inquisitor Drogan submits to the sorcerer's authority, as he had been the Chaos Lord's undead pawn all along. Nemeroth takes the Inquisitor's life, and cannot help but be intrigued by Titus' ability to resist his dark psychic powers, which indicates the Space Marine captain has a connection of his own to the Immaterium. Aiming to capture Drogan's experimental power source for himself, Nemeroth is interrupted by the Ork Warboss once more, who claims he is much harder to kill than the human Inquisitor. The two begin combat and tumble over the railing of the hive complex into the abyss.
Titus formulates a plan to destroy the spire where Drogan's weapon and the Warp portal is located using the unused Imperial Titan at the manufactorum. His plan goes unchallenged, but direct concern over Titus' obvious connection to the Warp unnerves Leandros, and it is not before being quickly silenced by Sidonus that he quiets his concerns. The three Astartes treturn to the Titan manufactorum, slaying any Orks or Chosen of Nemeroth Traitor Marines that get in the way while continuing to assist the Imperial Guard and Lt. Mira.
Once they arrive at the manufactorum, Titus is ambushed by the Ork Warboss Grimskull, who claims that no "humie" can ever defeat him. After a fierce battle, Titus blows the Ork leader's head to pieces. With the death of their Warboss, Mira announces that the Ork horde has started to fall back as its Nobs battle for control to see who will become the new Warboss, thus leaving the Chaotic forces as the primary concern for the Astartes.
Powering up the mighty Titan war machine with the experimental power source, Titus protects it while it is moved into position, and finally manages to launch a devastating strike at the hive city spire, which results in the closing of the Warp portal. Requesting extraction from his orbiting Strike Crusier via Leandros, Titus gives the experimental power source to Sidonus, instructing him to make for the extraction point while he and Leandros hold off the Chaos forces. Sidonus, however, is ambushed and killed by Nemeroth, who in turn informs Titus that Drogan had been dead for some time, and was simply animated as a puppet by his foul sorcery so that he could serve to draw the Space Marines to the planet, where they were used as the tool required to deliver the power source into the hands of the servants of the Dark Gods. With it, Nemeroth promises that he will soon ascend to become a Daemon Prince.
Titus, Leandros and Mira grieve for several moments over the fallen Sidonus. Titus instructs Leandros to turn his anger into rage to be directed towards the Forces of Chaos. Titus himself vows to slay the sorcerer for his crimes, and Mira advises him to use his strange resistance to the powers of the Warp as an advantage, killing Nemeroth before he has a chance to use the power source to ascend to become a new Daemon Prince.
Titus then begins his quest to avenge his fallen friend, and battles the Forces of Chaos with help from both the newly-arrived Blood Ravens Chapter of Space Marines and his own Ultramarines of the 2nd Company. Upon reaching the damaged hive city spire that served as the Chosen of Nemeroth's link to the Warp, Titus boards a Thunderhawk gunship and reaches the spire using a Jump Pack. Fighting his way to the top of the spire, he finally confronts Nemeroth, who insists that the connection to the Warp Titus has means that he is destined to fall to the corruption of Chaos and lead his own Ultramarines as Traitors in a new war against the Imperium. Titus, however, angrily refuses all such talk. He proclaims that his only destiny is to destroy all Traitors to the Emperor of Mankind.
Nemeroth manages to partially ascend to daemonhood, but Titus uses his resistance to the power of Chaos to launch himself at Nemeroth before he can complete his transformation, causing the two to fall from the spire towards the planet's surface at an alarming rate. During the fall, Titus uses all of his skill and fury to engage the nascent Daemon Prince in direct physical combat. After a brutal struggle, Titus finally smashes in Nemeroth's skull, resulting in the sorcerer's body melting back into the Warp.
Returning to the surface using his Jump Pack, Titus links up with his remaining Space Marines, Leandros and Lt. Mira. She assists Titus with his wounds, marveling in the realization that Space Marines truly are human after all. Titus proves happy to agree, because for all their enhancements and superhuman skill, as Astartes is still as mortal as any other man. The group is then interrupted by the arrival of a black Valkyrie bearing the symbol of the Inquisition, which drops members of the Black Templar Space Marine Chapter and Inquisitor Thrax.
Examining Titus, Thrax turns to Leandros and asks him if he is sure of the charge he is placing upon his captain, to which the young Space Marine informs the Inquisitor that Captain Titus has indeed been corrupted by Chaos, as no "true" Space Marine could interact with a link to the Warp as Titus had. The Inquistor then demands that Titus relinquish himself into his custody, stating that if he refused then the Inquisition would proceed to investigate all those who served under Titus's command, including Mira and her men. Titus steadfastly defends his honour, his actions and his fellow Astartes, saying that he does not know the meaning of his connection or resistance to the powers of the Warp, but knows that in his heart he is no Traitor to the Emperor.
Refusing to allow the Inquisition to torture those under his command, Titus agrees to surrender himself to Thrax, albeit with one final conversation with Leandros, who readily defends his position within the rigid guidelines of the Codex Astartes. Titus can only look on sadly, telling Leandros that while the Codex is a set of rules by which from which Space Marines should seek guidance, it is the manner in which you conduct yourself within those guidelines that truly makes one an Astartes, and by that measure, Leandros has failed to measure up to what it truly means to be a Space Marine.
Titus boards the Inquisitor's Valkyrie, surrounded by the Black Templars while his fellow Ultramarines look on with Lt. Mira. Leandros, feeling the guilt of his actions, turns and walks away in shame at his captain's words. Titus is taken off-planet to be investigated by the not-so-tender mercies of the Inquisition. Another Imperial cogitator screen then indicates that the threat to Graia has been contained and that the Inquisition will undertake a formal investigation of suspected heresy by Captain Titus.
Many large manufactoria dot Graia, as one would expect on a Forge World. Ajakis, the manufactorum Titus is sent to secure, is the rare facility capable of creating Warlord-class Titans, and the Graia Pattern Vengeance Launcher, a grenade launcher that can "stick" grenades to surfaces. The grenade can then be remotely detonated by the user. Graia also served as Inquisitor Drogan's testing ground for his Scourge weapon.