Ultramarines: A Warhammer 40,000 Movie - Plot Synopsis: A group of Space Marines of the Imperial Fists Chapter come under attack. A Space Marine by the name of Nidon is told to protect "the codex", and at that very moment a fireball engulfs the Astartes.Brother Proteus (Sean Pertwee) and Captain Severus (Terence Stamp) of the Ultramarines Chapter are locked in close combat onboard a Space Marine Strike Cruiser. As the other members of Ultima Squad watch, Proteus manages to disarm Severus and demands his surrender; however, Severus escapes Proteus' grasp and in turn defeats him, declaring that a Space Marine never yields. Afterwards the members of Ultima Squad are shown a sacred weapon of the Ultramarines, a relic Thunder Hammer, the Hammer of Macragge, that lies in their ship's Reclusium. The captain and the Apothecary Pythol (Donald Sumpter) lead the Ultima Initiates in a swearing-in ceremony in which they are sworn in on the Thunder Hammer, now ready for their first mission as Space Marines. When the ceremony is finished, Ultima Squad prepare themselves for their mission to the planet of Mithron.In transit to the planet's surface, Captain Severus addresses Ultima Squad, informing them that they are to respond to an astropathic distress call from Mithron that was received shortly before all contact with the planet was lost. A distress beacon is detected coming from the planet's surface but it is unknown whether it is automated. The only site of importance in the system is a shrine guarded by a full company of the Imperial Fists along with a garrison drawn from the Imperial Guard. This implies that if the defenders of Mithron are calling for help, matters are truly dire. Leaving the bulk of his company on Algol to continue an ongoing campaign, Severus sets off for Mithron with only the ten-man Ultima Squad for support. Accompanying the squad is Apothecary Pythol, a veteran of many battles and Severus' de facto advisor. In the squad itself, Battle-Brothers Proteus and Verenor (Steve Waddington) are the most eager to prove themselves in combat. On the tough and unforgiving surface of Mithron, Ultima Squad discovers that a terrible battle has taken place. The garrison force has been annihilated and the planet's Ecclesiarchy shrine has been desecrated. This makes it evident to the squad that theForces of Chaos are responsible for the attack. The squad decides they must continue and find the reason behind the distress beacon and discern if there are any remaining survivors.While approaching the ruins of the shrine, the Ultramarines are ambushed by Chaos Space Marines of a warband of the Black Legion. Three Ultramarines, including Sergeant Crastor, are slain, but the ambush is thwarted. The squad continues on into the ruined shrine but they are confronted by a Daemon Prince. The Daemon kills another Ultramarine and engages Captain Severus, and both combatants tumble over a cliff. With Severus gone and Sergeant Crastor dead, command of the squad falls to Proteus. While the rest of Ultima Squad wants to return to their Strike Cruiser and wait for reinforcements from their Chapter, Proteus decides to continue with the mission and find the source of the distress beacon. In the reliquary at the shrine's summit, they find Chaplain Carnak (John Hurt) and Brother Nidon (Johnny Harris), the sole surviving Imperial Fists. They reveal they have been protecting the Liber Mithrus, an ancient codex given to the Imperial Fists Legion at its Founding by the Emperor of Mankind Himself, and a sacred relic to them. Ultima Squad agrees to help Carnak and Nidon take the book to safety, but Verenor and Proteus remain suspicious, questioning how just the two of them have managed to survive for so long while every other person died.As Ultima Squad retreats to the extraction point, they are attacked by a massive force of Black Legion Traitor Marines. During the fight, Ultima Squad kills many of the attackers but suffer heavy casualties as well, being reduced to four Ultramarines, Apothecary Pythol, Chaplain Carnak, and Brother Nidon. Just as Ultima Squad is about to be overwhelmed, Captain Severus suddenly reappears and with his help, the remains of Ultima Squad, Carnak, and Nidon manage to escape back to their Strike Cruiser. Upon their return, Proteus confides to Severus about his suspicions concerning Carnak and Nidon and that he believes that they may have been tainted by Chaos. Proteus and Severus along with Brother Hypax (Gary Martin) confront the Imperial Fists, with Severus taking the book and discovering that it is blank. When Hypax enters with the Ultramarines standard, it ignites, indicating to the Astartes the presence of Chaos. Severus declares that Carnak has been tainted by Chaos. Carnak heavily objects to this, even accusing Severus himself of being the Traitor. Severus shoots and kills him. Nidon becomes enraged and attacks Severus but is easily thrown off and knocked unconscious. Hypax questions why the standard continued to burn with Carnak (the supposed Daemon Prince) dead. However, it is revealed that it is Severus that was possessed by the daemon that he fought. The possessed Severus grabs Proteus, Hypax then grabs the Chapter standard and charges the daemon, pushing it into the cruiser's Armoury. Hypax is last seen shutting the Armoury's door to protect Proteus. When Proteus and Nidon return to consciousness, they find Verenor standing over Hypax's dead body, who has been impaled by the very standard he had been entrusted with protecting. Under Verenor's leadership the three Astartes pursue the daemon, discovering the bodies of the remaining member of Ultima Squad and a heavily injured Pythol. Proteus, Verenor, and Nidon chase Severus and confront him in the starship's Reclusium. Severus' body can be seen to be mutating, rapidly developing daemonic features such as sharp teeth, horns and hooves. The daemon disables Nidon and Verenor and then seizes Proteus. It reveals that it plans to possess Proteus as its new Daemonhost, and while disguised as him, stow away on the Strike Cruiser to reach the Ultramarines' homeworld of Macragge, whereupon it will use the Imperial Fists' codex to open a Warp portal, allowing a horde of daemons to descend upon and destroy Macragge as happened on Mithron. However, Pythol arrives and intervenes, saving Proteus, but is killed by the daemon. Proteus tears the sacred Thunder Hammer from its housing in the Reclusium, while Verenor awakens and distracts the daemon. Proteus uses the Thunder Hammer to kill Severus and banish the daemon possessing him back into the Immaterium.Afterwards, Proteus is promoted to the rank of Sergeant with Verenor as his second in command, and both Astartes prepare to lead their own Ultramarine squad into battle. The final scene mirrors the opening one, showing the new Ultramarine recruits swearing on the same relic Thunder Hammer that Ultima Squad had sworn on and that Proteus had used to slay the daemon-possessed Severus.