When the TARDIS materialises on Earth in the year 2084, the Doctor meets an old enemy – the Sea Devils. Once the masters of this planet, they are now forced to live in the murky depths of the sea. But their intention is to reclaim their position of domination... This will entail the infiltration of Earth's defence systems and the provocation of another World War, more terrible than any yet experienced, to bring about the complete annihilation of the human race. Not only is the first stage of the Sea Devil's attack successful, their associates in this dastardly plan are the sinister Silurians, also known to the Doctor of old.