The film opens with a group of Space Marines of the Imperial Fists Chapter under attack from an unknown enemy. A Space Marine by the name of Nidon is told to protect "the Codex" and races to obey his orders, just before a fireball engulfs them all. Elsewhere, aboard a Space Marine Strike Cruiser, Brother Proteus and Captain Severus of the Ultramarines Chapter spar in a training duel. Proteus manages to disarm Severus, however, he quickly escapes Proteus's grasp and in turn defeats him, proclaiming that a Space Marine never yields. The members of Ultima Squad are then shown a sacred weapon in their ship's reclusium, a Relic Thunder Hammer. The Captain and his right-hand man, Apothecary Pythol lead the initiates in a swearing-in ceremony on the Hammer. With the ceremony finished, Ultima Squad prepare themselves for their first mission—a sortie to the planet of Mithron.