A Tyranid splinter fleet has invaded the Imperial system of Recidious. The Adeptus Mechanicus has called for aid to defend the jungle world of Kadaku and to safeguard the classified "Project Aurora". Demetrian Titus and his Deathwatch kill team are sent to defend Kadaku, but during the mission, Titus becomes the only survivor. Waking up aboard the Battle Barge, Chaplain Quintus reminds him that he was declared free of chaos taint by the Inquisition and if Titus seeks redemption, he must return to his chapter and regain their favor. After re-inducting him into the Ultramarines, Quintus orders Titus to keep his past a secret and to report to Sevastus Acheran, the new Captain. Acheran is skeptical of Titus' reinstatement, but gives him command of a squad, which includes Sergeant Gadriel and Brother Chairon. The team assists in a planet-wide evacuation of Kadaku, and rescues Archmagos Nozick, a senior member of the Adeptus Mechanicus, off-world, though the latter refuses to leave until data related to Project Aurora is retrieved from his research facility. While the operation is successful, the gunship transporting him crashes in Tyranid territory. Titus and his squad race to save Nozick but find him already dead. Surveying the wreckage, they discover that Nozick was not slain by Tyranids but by the Forces of Chaos. Before they leave, a squad of Ultramarines transport a mysterious object from the wreckage that causes Titus to lose consciousness. Acheran then orders Titus to go to Avarax to retrieve Nozick's apprentice, Morias Lueze, who can continue work on Project Aurora. They reach an Adeptus Mechanicus facility and Titus accesses a cogitator that gives him Lueze's location. Suspicious of the mysterious object, Titus searches the cogitator and discovers to his horror that Project Aurora is a weapon powered by the same chaos artifact Titus destroyed on Graia, retrieved and reconstructed by the Mechanicus. Titus returns to Battle Barge to confront Acheran, demanding that the project be stopped, but Acheran refuses as they are working under the orders of Roboute Guilliman, the primarch of the Ultramarines. Returning to Avarax, they reach Leuze and escort him to the Battle Barge. Gadriel's suspicion of Titus is raised after Lueze unwittingly reveals to them Titus' history with Chaos. Returning to Acheran, Titus urges him to contact Ultramarines Chapter Master Marneus Calgar and warn him about Project Aurora, though he was informed that the Astropathic Relay which would allow them to send a message to Calgar is currently overrun by Tyranids. Titus fights his way to the Relay, and meets with Astropath Neoma who offers to send a message to Calgar. As she sends the message, Neoma suddenly accuses Titus of being a traitor and trying to lure Calgar to the planet to kill him. A suspicious Gadriel attacks Titus but stops as Chairon shoots Neoma, recognizing her as corrupted by Chaos. Her body explodes in Warp energy revealing the Thousand Sons Sorcerer Lord Imurah. Imurah escapes and Titus returns to Acheran, who informs Titus that the Thousand Sons have appeared all over the Recidious System and are focusing on Demerium, home of Project Aurora where Lueze is attempting to activate the weapon. Titus and Gadriel make amends before they assault Demerium. They reach Lueze, who reveals that by combining Necron and Mechanicus technology with the Chaos artifact, Project Aurora will allow them to negate and banish all those touched by Chaos, across the entire galaxy. Lueze activates the machine and seemingly vaporizes Imurah, but he reappears and kills Lueze, revealing that he influenced the Mechanicus into creating Project Aurora. He steals the artifact and uses it to create a massive rift to the immaterium to bring more of his forces to Demerium. Reuniting with Acheran, the team attempted to close the rifts by destroying several obelisks that empower it. They are nearly overwhelmed, but are saved by the arrival of Calgar and his Ultramarine forces. During the battle, Calgar, Titus and his team enters the rift and battle Imurah. Titus takes the artifact while Imurah is distracted and destroys it, disintegrating Imurah. Returning to the Battle Barge, the Ultramarines celebrate their victory, and Calgar invites Titus to join him on a mission elsewhere. Before he departs, Titus is intercepted by Quintus, who reveals himself to be Leandros, Titus' former subordinate who had reported him to the Inquisition, and warns that he will eliminate Titus if he falls to corruption.