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In the fourth episode of the six-part story "Planet of the Spiders," the crooked Lupton (John Dearth) has stolen the powerful blue crystal coveted by the spider empress of the planet Metebelis 3. The empress' minions take over Lupton's mind, enabling the Doctor (Jon Pertwee), who brought the crystal to Earth in the first place, to determine that mankind will be destroyed unless the precious item is returned to its rightful owner. Meanwhile, Sarah (Katy Manning) is captured and transported to Metebelis 3 as a means of drawing the Doctor out. Written by Robert Sloman, "Planet of the Spiders, Episode 4" originally aired on May 25, 1974.