Mike Yates (Richard Franklin) is attending a meditation centre in 'darkest Mummerset'. He becomes suspicious of the activities of Lupton (John Dearth), Barnes (Christopher Burgess), Land (Carl Forgione), Keaver (Andrew Staines) and Moss (Terence Lodge), who seem to be summoning powers through their chanting. He arranges for Sarah to come along in her capacity as a journalist for Metropolitan magazine, to try and uncover what is happening. 

On their way from Mortimer station, Yates nearly crashes his car as Lupton's group makes him and Sarah see a tractor in the road ahead of them. At the centre, Sarah talks with Cho-je (Kevin Lindsay), the centre's Tibetan guide, and meets Lupton. Yates hurries her away but they return on foot. After persuading Tommy (John Kane), a simpleton who works at the centre, to keep quiet, they hide in the cellar before Lupton's group arrive and start chanting. To Sarah's horror, a giant spider appears before them. It jumps on Lupton's back and vanishes. 

Meanwhile the Doctor has made contact with stage magician Professor Clegg (Cyril Shaps) who possesses genuine powers. Clegg demonstrates telekinesis to the Doctor and the Brigadier (Nicholas Courtney). When given the Doctor's sonic screwdriver he 'sees' Drashigs and when Sergeant Benton (John Levene) arrives with a package from the Amazon, Clegg correctly identifies its contents as the blue Metebelis crystal given to Jo as a wedding present. While the others are distracted, Clegg looks into the crystal and is killed. 

Sarah returns to UNIT and tells the Doctor what happened. When she mentions a spider, the Doctor is immediately interested - spiders were the last thing seen by Clegg before he died. 

Lupton arrives at UNIT HQ in search of the crystal, as instructed by the spider (voice: Ysanne Churchman). He knocks out a soldier (Pat Gorman) with powers given him by the spider on his back. He sees the crystal in the UNIT lab and transports it to his hands. Sarah, the Doctor, Benton and the Brigadier give chase. After a chase involving the Doctor's new space-car, Bessie, an autogyro, a speedboat and a hovercraft, the Doctor catches up with Lupton only to find that he has vanished. 

Lupton materialises at the meditation centre, watched by Tommy, who likes the crystal. As Lupton and Barnes discuss what to do, Tommy steals the crystal. The Doctor arrives to see Cho-je. Tommy wants to give Sarah the crystal as a present but before he can she overhears Lupton planning to perform the ceremony in the cellar on his own and tells Tommy to alert the Doctor. She watches as Lupton vanishes and as the Doctor rushes into the cellar, she vanishes as well and reappears on Metebelis 3. 

There she meets Nesca (Jenny Laird) and Sabor (Geoffrey Morris) and their children Rega (Joanna Monro), Tuar (Ralph Arliss) and Arak (Gareth Hunt). Tuar wants to kill her but Arak stops him. Huath, the Queen of the eight legs (voice: Kismet Delgado), arrives and Sarah is hidden in a hut. The spider Queen wants Arak surrendered as he killed a guard. Sabor offers himself in Arak's place but Sarah is seen and is held by the Queen's guards. The Doctor arrives in the TARDIS and fights with the guards (guard captains: Walter Randall, Max Faulkner; guards: Leslie Bates, John Cash, Harry Fielder, Derek Hunt, Steve Ismay, Roy Pearce, Dennis Plenty, Geoffrey Witherick). In the confusion, Sarah is again hidden by Nesca. The Doctor is knocked out and, when the Queen has gone, he is taken into Nesca's hut. He is very weak and tells Sarah to fetch a machine from the TARDIS which will help him. She gets the machine, but is captured by guards and taken away. Arak retrieves the machine from by the TARDIS and the Doctor uses it to save his own life. 

Meanwhile, Tommy has been listening as Barnes and the others wonder where Lupton has gone. Tommy is caught and goes to practice his reading. He lays his collection of 'pretty things' around him and starts to read. He finds himself looking into the blue crystal and collapses. When he recovers he finds he can read with ease. 

Arak explains how the spiders came to Metebelis 3 - they arrived with the humans on a starship 433 years ago. The spiders were blown into the mountains where the blue crystals enlarged their minds and bodies until they were powerful enough to enslave the human population. The Doctor sends Tuar and Arak to collect pebbles for him and discovers that among them is one that will protect the wearer from the spiders' energy blasts. 

The Doctor goes to the spiders' city but is captured and placed in the 'larder' with Sarah and Sabor. Sarah is released and taken before the Queen spider. She tells Sarah that she disagrees with the plan to invade Earth and wants to help. Sarah believes her and agrees to help. 

The Doctor frees himself from the 'larder' and hears Sarah calling to him. He follows her voice to a cave of blue crystal. It is the Great One playing a trick on him. The Great One (voice: Maureen Morris), the ruler of all the spiders, wants the Doctor to return to Earth and fetch the blue crystal for her. 

On Earth, Yates talks with Barnes and persuades him to reopen the link in case Lupton is waiting for this to return to Earth. They meet in the cellar and start chanting. Scout spiders appear in the shadows behind them. 

Tommy tells Cho-je all he knows. Cho-je heads for the cellar but a spider blasts him and Yates before other spiders attach themselves to Barnes, Moss, Land and Keaver. Tommy hurries away to warn the Abbot, K'anpo Rimpoche. 

The Doctor finds Sarah and she transports him back to the village where they enter the TARDIS and return to Earth. They are attacked by the spider controlled humans but the Doctor has a Metebelis stone which deflects the blasts. Tommy takes them to see the Abbot. 

The Doctor recognises the Abbot (George Cormack) as his old Time Lord tutor. Tommy holds the others off at the door, absorbing the energy blasts as he is innocent. When K'anpo brings out the crystal Sarah demands it. She is controlled by the Queen spider. The Doctor uses the crystal to free Sarah from the spider's influence and it falls from her back and dies. K'anpo reveals that Cho-je is a projection of himself. He makes the Doctor realise that this is all his fault as he took the crystal in the first place, and that he must now face his fear and return to the cave of the Great One to give back the crystal. 

The spiders summon up more power and blast Tommy again. Yates arrives and also takes a blast, but Barnes and the others get into K'anpo's room and attack the Abbot who regenerates into the form of Cho-je. The Doctor vanishes and reappears in the cellar. He returns in the TARDIS to Metebelis 3 where he meets Arak and Tuar who take him to the spider's council chamber - their attack failed and they are under the spiders' influence. The Doctor tells them that he is going to return the crystal to the Great One and Lupton tries to stop him. As the Doctor leaves, Lupton is killed by a blast from the spiders. 

The Doctor arrives at the cave of the Great One and confronts the giant spider. She takes the crystal from him and completes her 'web of power' which is, the Doctor realises, a positive feedback circuit that will eventually destroy the Great One's mind. The Doctor is getting weaker from the radiation but races from the chamber and manages to leave in the TARDIS before all the spiders die, the humans are released from the spiders' mental domination and the mountain explodes, destroying the Great One. 

Three weeks later, Sarah is at UNIT HQ with the Brigadier wondering if they will ever see the Doctor again when the TARDIS appears. The Doctor got lost in the time vortex and the TARDIS has brought him home. He collapses on the floor and apparently dies. Cho-je appears and tells the Brigadier and Sarah that he will assist the Doctor's regeneration. He does so and the Doctor's appearance changes before their eyes.

_{Synopsis from Doctor Who: The Third Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}