On a small Imperial frontier world, two forces clash in vicious battle as their starfleets play a deadly game of cat and mouse in the outer solar system. The Tau landed months ago, laying waste to planetary defense forces, with the notable exception of Maldein, the hive capital. Their agents work to convert the population of Sagalein to the Path of the Greater Good, the infamous Tau’Va. They’ve also stumbled upon something strange and ancient at one of the poles, an installation with a purpose defying identification.

The Tau weren’t the first to stumble upon it. Techmarine Master Spurius Lentullus of the Ultramarines responded to a call by agents of the Omnissiah on Sagalein during the Tau invasion. Together with his hard-charging subordinate Ultramarine Lieutenant Plautus and Colonel Arslan, the Lion of Macabbius Quintus, Lentullus fights to reclaim the lost mysteries of Sagalein in the name of the Omnissiah and free the planet from xeno dominion without revealing his true purpose.

Stuck in the middle are the soldiers of the Marshot 39th Infantry, forced to surrender during the Tau onslaught and now fighting for the xeno, Field Marshall Zacair Fetaine, the disgraced commander of the Planetary Defense Force still desperately trying to hold his people together, and millions of Sagaleiners forced to choose between loyalty to the far-distant Emperor of Man and the better life promised by Tau diplomats.

Duty is never simple, and the line between right and wrong has never been harder to see…