During one of Nyssa's experiments, the TARDIS's temporal scanner picks up a message: 'Idra'. Just one word, but enough to draw the Doctor to the Archipelago of Sirius. There, the Autarch is about to announce a new crusade. A mighty war against Seth, Prince of the Dark. But who is Seth? What is the secret of Queen Anahita, Mistress of the Poisons? And what terror awaits on Level 14?