The squad gets their briefing for their next mission; to locate a Chaos Space Marine who has escaped the custody of the traitorous rebels. The ancient and twisted Marine, Warpsmith Arkturian, has been responsible for the corrupting spread of Chaos through the star fort. He is currently working his evil technomancy on the heart of Lycurgos, attempting to convert the entire place into a massive daemon engine, a result which would be even more disastrous than allowing the facility to stay in the hands of the Severan Dominate.

Due to their past experience fighting the rebels and the forces of Chaos, Alpha Squad is given a lead role in this critical mission and full access to the stores of the 1st Aerial Assault Regiment on the star fort to arm themselves.

Acquiring a powerful Stormlord super-heavy tank, meltaguns, and some suits of carapace armor to help them confront their deadly foe, Alpha sets out at the head of A Company as it plunges into the depths of the imperiled fortress. They also come into accidental possession of a unmarked crate containing a jeweled box with the seal of the Inquisition. Sergeant Arion opens it and finds that it contains an ancient parchment map showing the layout of the fortress, overlaid with bloody scrawls indicating places of Chaotic power. The Squad uses this blasphemous map to help them avoid the likely spots of enemy ambushes, but it results in a longer trip through the fort corridors.

The troopers encounter a labyrinthine series of new passages constructed by Chaos-tainted servitors on their way to the Enginarium. They battle against waves of cultists, enemy armor, and Daemon Engines, suffering only minor damage to their powerful Stormlord as they press forwards. Reaching their destination at the center of the star fort, most of Alpha dismounts from the tank and seeks out the Warpsmith among the catwalks and platforms of the massive Enginarium. They encounter an old Tech-Priest as they search, and he fills them in on Arkturian's plan to start his ritual soon. Sergeant Arion and Yekaterina are first to find the hulking armored Chaos Space Marine, and a vicious battle begins between the troopers and the mad superhuman.

Many members of A Company are slain by Arkturian's assortment of weapons, and Yekaterina is nearly killed by his flamer. Sergeant Arion retreats from the Marine's assault and attempts to reform the scattering, terrified troopers once more.

Realizing that their infantry weapons are insufficient to stop the Warpsmith, he retreats to a level where the Stormlord tank will be able to target the pursuing enemy. Darrien sights Arkturian as he chases the guardsmen and lays into him with the tank's cannons. Even these attacks are not enough to kill the Warpsmith, so Arion manages to get a heavy weapons squad together and the combined power of their meltaguns melts the raging Arkturian into a puddle of sickening goo.

While the battle for Lycurgos is not over immediately, Alpha Squad has defeated the leader of the Chaos forces and captured the general of the rebels. Holding their ground at the Enginarium, they continue the fight of the Imperial Guard to wipe out the remaining enemies. The battle is hard and the sacrifices costly, but after several more engagements, the 1st Aerial Assault Chrysopelea and what's left of the Vostroyan Void Shrikes regiment win the day – Lycurgos is once more in the possession of the God-Emperor.