The Horus Heresy reaches the Solar System - will the defenses of Rogal Dorn and his Imperial Fists stand against the onslaught of enemies? Recalled from the fields of the Great Crusade after Ullanor, Rogal Dorn and his Legion were appointed Praetorians of the Emperor, but it was only after the Warmaster's treachery was revealed that the full extent of the sacred duty entrusted to them became apparent. Now, the Solar System is under attack for the first time since the war began, and the many seemingly impenetrable fortifications prepared by the Imperial Fists Legion are proving inadequate. All eyes are on this new threat coming from beyond the Gates of Terra, but who will protect the Primarch of Dorn from his enemies within?