Following the events of Invasion of the Dinosaurs, Mike Yates was discharged from UNIT and is now attending a Tibetan meditation centre in rural England. He is visited by Sarah Jane Smith and they witness some curious happenings at the centre, seemingly organised by a resident called Lupton, a middle aged former salesman, and his cronies. Mike and Sarah stumble across Lupton performing an incantation, which conjures up a giant spider into the middle of the basement room. It jumps on Lupton’s back and then disappears. The spider manifests itself in Lupton’s head, telling him to seek out and locate a certain blue crystal.
The Third Doctor has developed an interest in psychic ability, but his testing of a clairvoyant called Professor Clegg backfires when his subject has a heart attack. It is triggered when Clegg comes into contact with a blue crystal from Metebelis Three (sent back from the Amazon by Jo Grant), which caused him to see the image of deadly spiders. Sarah returns from the retreat, having left Mike to watch things there, and she and the Doctor swap spider tales. Meanwhile Lupton has also arrived at UNIT HQ and steals the crystal from the Doctor’s laboratory. A multi-vehicle chase ensues which Lupton escapes by teleporting himself back to the monastery. Once there, the spider reveals that it is plotting against some of its sisters back on Metebelis Three. The spiders and the crystal originate from the same blue planet in the Acteon Galaxy, which was none too hospitable to the Doctor the last time he visited (during The Green Death).
The Doctor and Sarah now make for the monastery and tell the deputy abbot, Cho-Je, that something is very amiss. The crystal now strays again when it is taken by Tommy, the simple-minded handyman of the retreat, whose mind is opened and improved by the power of the crystal. Lupton is teleported to Metebelis Three, unconsciously allowing Sarah to follow him. She soon meets the human slave inhabitants of the planet, a generally dispirited bunch, other than the rebellious Arak, who flees to the mountains.