The story made up of episodes 1-4 is generally referred to as The Mysterious Planet.

The TARDIS is drawn down into a large space station, where the Doctor, appearing rather dazed, exits alone from his ship. His memory of recent events has apparently gone, along with his companion Peri. He steps through a doorway to find himself in a courtroom. Opposite him is the Valeyard (Michael Jayston), who chides the Doctor for being late. The court stands as the Inquisitor (Lynda Bellingham) enters and takes her place. 

The Valeyard, the prosecuting Time Lord, explains that this is an impartial inquiry into the Doctor's behaviour, which, he hopes to prove, is unbecoming of a Time Lord. Furthermore, he intends to prove that the first law of time has been broken. 

The Valeyard draws on information from the Matrix to show the Inquisitor and the Time Lord jurors a series of events from the Doctor's recent past. On a large screen set in the back wall of the courtroom, the Doctor sees himself and Peri walking on the surface of the planet Ravolox in the Stellian galaxy. 

Ravolox was supposed to have been destroyed by a solar fireball five centuries ago, but the lush vegetation belies this fact. It seems that the Gallifreyan records are wrong. The Doctor also notes that Ravolox is almost identical to Earth - a fact which puzzles him. The mystery deepens as Peri affirms that the planet feels just like Earth. The Doctor finds a necklace in the undergrowth, proving that they are not alone here. 

The travellers' progress is observed by the mercenary Sabalom Glitz (Tony Selby) and his sidekick Dibber (Glen Murphy), who are attempting to destroy a Magnum Mark 7 Light Converter - a black light collection aerial - which is supplying energy to an L3 robot. They decide to kill the Doctor and Peri, but the attempt is foiled when the pair duck out of sight. 

The Doctor and Peri have entered an old building which turns out to be the remains of Marble Arch station in the London Underground - apparent proof that Ravolox is in fact the Earth, two billion years or so after Peri's time. However, the planet and its constellation have been shifted two light years across space. 

Finding a hermetically sealed door which leads further down into the building, the Doctor resolves to explore. Peri, upset at the fate of her planet, opts to stay on the surface, but soon regrets her decision as she is captured by a group of primitive-looking humanoids. 

Down in the brightly lit tunnels, the Doctor picks up a carafe of water and is immediately arrested for being a water thief. Balazar (Adam Blackwood), keeper of the three Books of Knowledge - Moby Dick, The Water Babies and UK Habitats of the Canadian Goose - orders that the Doctor be stoned to death. The Doctor's presence has also been noted by the Immortal - the L3 robot - and it sends the train guard leader Merdeen (Tom Chadbon) to investigate. Meanwhile, the stoning commences and the Doctor is knocked unconscious. 

The Valeyard now suggests that the inquiry be turned into a trial. If the Doctor is found guilty, then his life will be terminated. 

Glitz and Dibber encounter a group of the primitive-looking natives, members of the Tribe of the Free, and are taken to their leader, Queen Katryca (Joan Sims). The matriarch explains that the black light collection ariel is a totem to the earth-god Haldron, worshipped by the Tribe, and that many others have come before, all with different stories as to why it should be destroyed. She orders that Glitz and Dibber be held in a guarded hut along with Peri. 

The Immortal orders that the Doctor be brought to him. The Doctor discovers that the robot is called Drathro (voiced by Roger Brierley), and that its purpose is to guard three Sleepers from Andromeda, along with the secrets in their possession, until such time as they can be returned to Andromeda. The black light converter has now developed a fault, however, and without power the Sleepers will die. Drathro wants the Doctor to repair the converter. The Doctor tricks Drathro and his helpers, Humker (Billy McColl) and Tandrell (Siôn Tudor Owen), and manages to escape from the control room. Drathro sends an L1 service robot after him. 

The Doctor discovers that Merdeen has been smuggling people out of the city so that they can avoid the regular culling which takes place to keep down their population's numbers. It is generally believed amongst the city-dwellers that the planet's surface is uninhabitable, but Merdeen knows this to be untrue. The Doctor and Balazar head for the surface. 

Glitz, Dibber and Peri escape from their make-shift cell, and Dibber destroys the collection ariel. They meet up with the Doctor and Balazar at Marb Station but become trapped between the pursuing Tribe and the service robot. One of the Tribe, Broken Tooth (David Rodigan), recognises Balazar as an old friend and attacks the robot, temporarily halting it. Everyone then returns to the Tribe's village, where Katryca orders that Glitz, Dibber and Peri be taken back to the prison hut, this time joined by the Doctor. 

The reactivated service robot subsequently crashes through the hut wall and seizes the Doctor, but the Tribe attack and disable it. Mistakenly assuming that it was the Immortal, the emboldened Katryca mounts an attack on the station with her warriors. She and Broken Tooth are both killed by Drathro, while Glitz and Dibber make their way into the station to try to obtain the Sleepers' stolen secrets. 

A section of the evidence is bleeped out at this point, and the Inquisitor wants to know why. The Valeyard explains that it is because the information is sensitive and has no bearing on the Doctor's trial. The affected portion concerns the origin of the secrets. 

The Doctor gains access to Drathro's control room and is soon joined by Peri, Merdeen, Glitz and Dibber. Glitz manages to persuade Drathro to accompany him to his ship with the secrets, claiming that he will return them to Andromeda. When they have gone, the Doctor tries to shut down the converter and thereby prevent a chain reaction which would result in the destruction of the universe. He succeeds in his main aim, but the control room is wrecked in an explosion. En route to Glitz's ship, Drathro is affected by the blast and collapses, burnt out from inside. The heat also destroys the secrets. Dibber consoles the disappointed Glitz by pointing out that the black light aerial was made from siligtone, the hardest and most expensive metal in the galaxy, which means that he can still come away with a sizeable profit. 

In the courtroom, the Doctor insists that his interference was justified in order to save the universe from destruction. Yet many questions remain unanswered. Who were the mysterious Sleepers, and what secrets had they stolen? Who had moved Earth through space and changed Gallifrey's records to make it appear that the planet was Ravolox? The Doctor determines to find out.

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