The Doctor discovers the TARDIS has fallen into E-Space, an alternate universe, and landed on the planet Alzarius. The Doctor and Romana find its only inhabitants are living on a vast, crashed spaceship they have been repairing for generations to return to their home planet, Terradon. But Mistfall, a mythical time of terror, is coming again to Alzarius, and an eerie menace is rising out of the misty marshes. But why have huge poisonous spiders begun to appear in the Alzarian food supply? And what is the secret of the strange marshmen? The Doctor and Romana must solves these riddles and more if they are to have any chance of returning to their own universe.
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