Plot
 | This episode's plot summary may be too long or excessively detailed.(June 2017) |
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. Sarah Jane Smith visits him and they witness some curious events, seemingly organised by a resident called Lupton, and his cronies. Mike and Sarah stumble across Lupton performing an incantation, which conjures up a giant spider. 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 fatal 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, 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, with vehicles including Bessie, the Doctor’s Edwardian roadster; a Campbell Super Cricket gyrocopter, G-AXVK; the Whomobile, a futuristic-looking vehicle that combined the abilities of a hovercraft with the ability to fly; and a conventional hovercraft. Lupton escapes by teleporting back to the monastery. 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, Metebelis Three, in the Acteon Galaxy the Doctor last visited during The Green Death .
The Doctor and Sarah make for the monastery and tell the deputy abbot, Cho-je, that something is very amiss. The crystal is taken by Tommy, a handyman with learning difficulties, whose mind is opened by the crystal's power. Lupton is teleported to Metebelis Three, unwittingly allowing Sarah to follow him. She meets the human slave inhabitants of the planet, a generally dispirited bunch, other than the rebellious Arak, who is in hiding.
The planet is ruled by giant spiders who style themselves "the Eight-Legs" and their Queen is the supreme ruler. They govern using guards chosen from among the planet's Two-Leg (human) population and their own phenomenal mental powers, amplified by the blue stones of the planet. The Doctor arrives and meets Arak, who explains that the Metebelians are the descendants of the crew of an Earth space ship, which crashed hundreds of years before. A spider on board found its way to the Blue Mountains where, through the effect of the crystals, its progeny grew larger and cleverer. The Doctor works out that a “negative” stone can absorb and reject the power of the blue crystals. He starts a revolt among the humans, now protected by these stones, but this is not effective. The Doctor explores the Blue Mountains and encounters the Great One, an enormous spider that controls Metebelis and desires power over other domains, too. She craves the crystal, as it will complete a lattice of hundreds of similar crystals on her web and magnify her mental powers immensely. She knows the crystal is still on Earth and sends the Doctor there to get it for her. He flees back to Earth with Sarah, not knowing the Queen spider has now implanted itself in his companion's mind.
Tommy has given the crystal to the abbot, K’anpo Rimpoche, who is now shown to be an elderly Time Lord and the one-time hermit mentor of the Doctor. He now lives in peaceful exile on Earth. He tells the Doctor of Sarah's control, and they work together to expel the Queen Spider. A fight breaks out in the monastery between Lupton’s cronies and Tommy. The Abbot advises the Doctor to take the crystal to the Great One. He suggests that, as the Doctor started this chain of events by removing the crystal in the first place, it is up to him to put it back. He departs in the TARDIS with the crystal.
On Metebelis Three, Lupton is killed by the spiders after falling out with the council. When the TARDIS lands, the Doctor heads to the cave of the Great One with the crystal. He warns her of the danger of completing the lattice but she does so anyway. The forces unleashed are too strong for the Great One, and the positive feedback causes agonizing deaths to her and the other spiders. A vast wave of deadly radiation floods the cave. The Doctor, now very weak, staggers back to the TARDIS.
Weeks later, Brigadier Lethbridge-Stewart and Sarah are in the Doctor's laboratory when the TARDIS materialises, and on exiting it the Doctor collapses on the floor. The abbot K’anpo arrives in his new body, having regenerated into the form of Cho-je, who was a sort of forward projection of his soul. He tells them that the Doctor will change too and before their eyes the Doctor regenerates into his fourth incarnation.
