During one of Nyssa’s experiments, the TARDIS’ temporal scanner picks up a message: “Idra”. Just one word, but enough to draw the Fifth 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?