The Earth Empire — the Imperium Humanum, upon which a thousand suns never set — is dying. The Great Houses of the Empire manoeuvre and scheme for advantage; alliances are made; and knives flash in the shadows. Out among the moons of Jupiter, another battle is just beginning, as an ancient brotherhood seeks limitless power and long-overdue revenge. The Doctor returns to the thirtieth century, searching for the source of a terrifying weapon. He fears a nightmare from his own past may be about to destroy the future. Nothing must be allowed to get in his way.