The Doctor and the Brigader are attending a boring lecture on Eastern religion at a Tibetan meditation centre. In the basement, men are chanting. A mandala on the floor begins to glow, but Mike Yates (Richard Franklin) blunders in on them, breaking their concentration. After the lecture, the Doctor talks to Professor Clegg (Cyril Shaps) about ESP. The nervous professor claims to be a trickster, but the Doctor knows he’s a genuine clairvoyant. He browbeats the professor into moving objects with his mind. Sarah Jane Smith arrives on a story and is met by Mike Yates, who is investigating otherworldly matters after his dismissal from UNIT left him rootless. Sarah and Mike nearly have an accident caused by a non-existent tractor. Professor Clegg is tested holding the Doctor’s sonic screwdriver and sees monsters from the Doctor’s past. He also identifies a sealed package from Jo Grant in South America as containing a blue crystal from Metebelis Three. It causes him to see giant spiders and he has a heart attack. In the basement, the chanters have raised a giant spider.
A shady businessman named Lupton (John Dearth) arrives and the spider attaches itself invisibly to his back. He steals the crystal and tries to flee in the Whomobile. There is a long chase involving five vehicles that uses up a great deal of Chapter Two, and at the end Lufton is transported back to the centre by the spider. Her sisters on Metebelis Three are planning the conquest of Earth. Tommy, a mentally challenged handyman at the centre, steals the shiny blue object and tries to give it to Sarah Jane, but she pays as little attention to him as everyone else, so he keeps it in a box in his closet. It begins to enhance his intelligence and the longer he keeps it the smarter he gets. Lufton goes to the basement, followed by a suspicious Sarah Jane. His chanting transports him to Metebelis Three, and Sarah is pulled along behind him, where he meets the Eight Legs and their Queen, and she falls in with a family of slaves descended from the crew of an Earth ship that crashed hundreds of years ago. A spider on board made its way into the blue mountains, where the crystals mutated them into giant, intelligent spiders.
The Doctor goes to the Blue Mountains and meets the Great One—the biggest spider of them all and quite insane. She wants the crystal to complete a device which will give her galactic power. Lufton tells the Spider Court that he knows where the crystal is hidden, but he doesn’t. Tommy still has it and is reading Blake. But he gives it to the abbot K’anpo Rimpoche (George Cormack). Everyone ends up back in the house—the Doctor and Sarah, Lupton, and a horde of spiders. The Queen Spider is controlling Sarah. But the abbot gives the crystal to the Doctor, who recognizes him as an ancient guru and former Time-Lord, who frees Sarah, Tommy defends the room from all the bad guys as long as he can, but they break in and kill the abbot.
The Abbot has told the Doctor to return the crystal to the Great One, since it was his walking off with it that started all this. He takes off in the TARDIS to do just that, but the Great One destroys herself in the process of gaining total power. Lufton is killed by the other spiders, the slaves are freed, and the Doctor heads home. It takes him three weeks, Sarah and the Brigadier believe he is dead, but the TARDIS materializes, he steps out and collapses. The Time-Lord abbot, in a new body, appears for a moment, tells them the Doctor may seem a little strange now, and Jon Pertwee changes into Tom Baker.
Producer Barry Letts, who was fascinated with Buddhism, and script-editor Terrance Dicks were leaving. Katy Manning had already left, and Master Roger Delgado, whose planned swansong this story replaced, had died suddenly. Jon Pertwee, according to the tale, asked for a raise, worrying about being type-cast, and had been refused summarily. So, he quit too. There are some who doubt this. Mike Yates redeemed himself after his traitorous actions of the last few stories. Ysanne Churchman, who had provided the voice of Alpha Centauri in the Peladon stories, voiced Lupton’s spider. Kismet Delgado, Roger’s widow, voiced the Queen Spider.