The Doctor receives a mysterious package from his old companion, Jo Grant.
In the English countryside, Mike Yates, discharged from UNIT and recuperating at a rural Buddhist meditation center, is concerned about a small group who meet secretly in the basement; he suspects they may be attempting to summon something evil through their meditation. Meanwhile, the Doctor is researching psychic energy and enlists the assistance of a clairvoyant, Professor Herbert Clegg.
His experiments backfire when Clegg’s fixation on the Metebelis crystal (which Jo Grant had recently returned to the Doctor) leads to his seeing giant spiders, and he dies of a fright-induced heart attack. Mike enlists the assistance of Sarah Jane, whose initial skepticism is erased after they are nearly run off the road by a hallucinatory tractor. Sneaking into the basement they see the group, led by Lupton, summon a giant spider (similar to the ones Clegg saw) into existence.