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In the conclusion of the four-part story "The Twin Dilemma," the Doctor (Colin Baker) must prevent the giant slug Mestor (Edwin Richfield) from polluting the universe with millions of Gastropod eggs. But will the Doctor; his companion, Peri (Nicola Bryant); and twin mathematicians Romulus and Remus Sylvest (Gavin and Andrew Conrad) manage to live through the ordeal? And how does the Doctor's fellow Time Lord, Azmael, fit into the proceedings? Written by Antony Steven, Doctor Who: The Twin Dilemma, Episode 4 was originally telecast on March 30, 1984, bringing season 21 of Doctor Who to a close.