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In the conclusion of the four-part story "Warriors of the Deep," the Doctor (Peter Davison) has exposed a scheme hatched by the sea-dwelling Silurians to destroy all mankind, using a 21st century Cold War as their cover and an electrically charged beast called the Myrka as the main instrument of destruction. The Doctor's only hope rests with several canisters of hexachromite, which is lethal to the Silurians and all similar races -- but he is reluctant to unleash so deadly a chemical on the unsuspecting world. Originally telecast January 13, 1984, "Warriors of the Deep, Episode 4" was written by Johnny Byrne.