While the Warmaster's rebellious forces sweep across the galaxy, behind the scenes of the Imperium, a completely different kind of war rages. Knights Errant, chosen by Malcador himself, operate secretly in dark corners where others cannot. Seeking another kindred soul for his elite squad of warriors, Battle Captain Nathaniel Garro heads to the Phalanx, the mighty space fortress of the Imperial Fists.

Returning from the battlefields of Calth, Nathaniel Garro, Knight Errant and agent of the Sigillite, discovers that a new task awaits him - a battered flotilla of space marines from several Legions, located on the edge of the Terran System. With the presence of the World Eaters and the Emperor's Children causing wariness among loyalist defenders, and the grim parallels drawn to his own turbulent flight, Garro must look deeper, beneath the surface of the obvious, if he is to discern who is friend and who is foe...

Although war is raging throughout the Galaxy, for many people on the surface of the Throne World of Terra it exists only in the form of rumors and rumors. However, the humble clerk-adept Katano Talleri, in the course of her daily duties, discovers evidence of a conspiracy within the colossus of the Administratum. Of course, this can only portend one thing: a treacherous blow to the Solar System itself! Tallery flees into the shadows and soon finds himself in the hands of the mysterious Knight-Errant Nathaniel Garro. Who will this valiant defender of humanity become for her - a savior or an executioner?