At a Tibetan retreat in the English countryside, a group of men are using ancient meditation rituals to tap into a mysterious alien power. They unwittingly create a bridgehead between Earth and Metebelis 3, a planet where the 'Two Legs' are oppressed by giant spiders. The creatures are desperate to recover the blue jewel that the Doctor gave to Jo Grant as a wedding gift. Now the Doctor must risk everything and return to Metebelis 3 to face the awesome power of The Great One, who plans to use the great crystal to enslave all of human kind.