The Doctor's past catches up with him when he carries out an experiment with a blue crystal, which he stole from Metebelis 3. The crystal is also sought by the Eight Legs, a race of mutated spiders, as the final element in their plan for universal domination. 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 crystal to enslave all of human kind.