At the heart of the ruined city of Arcopolis is the Fortress. It's a brutal structure placed here by one of the sides in a devastating intergalactic war that's long ended. Fifteen years ago, the entire population of the planet was killed in an instant by the weapon housed deep in the heart of the Fortress. Now only the ghosts remain. When The Doctor arrives, determined to fight his way past the Fortress's automatic defenses and put the weapon beyond use, he soon discovers he's not the only person in Arcopolis! What is the true nature of the weapon? Is the planet really haunted? Who are the Eyeless? And what will happen if they get to the weapon before the Doctor?