The Mysterious Planet ( TV story ) The Mysterious Planet was the unbroadcast title given to the first four episodes of " The Trial of a Time Lord " , the season - long story that constituted season 23 of " Doctor Who " . The story marked the first appearance of the Inquisitor and the Valeyard , two characters who would appear throughout the season , along with Sabalom Glitz , who would appear again later in the season and also in Season 24 . With this chapter , the series returned to half - hour episodes . Also beginning with this story , the series was now completely produced on videotape ( with the exception , in this story , of a brief special effects sequence in Episode 1 ) . With this episode , Peter Howell ' s rendition of the " Doctor Who " theme was exchanged for a mysterious and surreal arrangement of the theme tune composed by Dominic Glynn . It would remain in use solely for season 23 and close out Colin Baker ' s era on the series . This story also introduced some costume changes for the Doctor & Peri . The Sixth Doctor entered his second ( and last ) full season with a red check vest & cravat and a bright pink watch chain . To help viewers discern between scenes taking place in different points in the Doctor ' s timeline , scenes set in his past feature him donning his original turquoise cravat & neon green watch chain for the last time ; the following story would feature the Doctor ' s full new outfit in both past and present scenes , and the one after that featured a completely different vest & cravat for the archival scenes , which are set in the Doctor ' s " future " . Notably , Nicola Bryant ' s character Peri Brown starts wearing more conservative clothing from here out , after spending most of her appearances in loose - fitting and often revealing outfits . Her wardrobe change was a result of complaints that her provocative clothes were inappropriate for younger " Doctor Who " viewers . Synopsis . The Doctor is summoned before the High Council of Time Lord s to stand trial for the charges of harmful interference to the course of events during his space - time excursions , which have threatened the sanctity of the universe . Indignant at these accusations , the Doctor pleads his case to the Inquisitor with the hope that she will see him as a source of hope and goodwill for existence . However , his prosecuting attorney , a sinister Time Lord known simply as the Valeyard , begins a crusade against the Doctor ' s life with the motive of painting him out to be a villainous renegade . The Valeyard ' s first movement against the Doctor is to review his past interactions on a familiar planet called Ravolox , where he and his - then companion Peri met the morally grey Sabalom Glitz and a tyrannical robot stalking the world ' s desolated landscape . However , Ravolox holds a terrible truth in the far reaches of its ruins , while the Doctor ' s trial has its own fair share of startling twists and turns . . . Plot . Part one . The TARDIS materialises in a corridor , and the Sixth Doctor emerges , bewildered and alone . He walks into a room , where he is put on trial for conduct unbecoming a Time Lord . The Inquisitor notes that the Doctor has been on trial previously . The Valeyard states he will argue that the Doctor was shown too much leniency on that occasion . The Valeyard opens the case by using the Matrix to show the Doctor ' s actions on the planet Ravolox . The Doctor and Peri arrive on Ravolox , which is virtually identical to Earth . He tells Peri that the official records state that the planet was devastated by a fireball , but the forest they are walking through suggests otherwise . They are seen by Sabalom Glitz and Dibber , who attempt to shoot the Doctor . He moves off just in time . Glitz and Dibber discuss their plan to destroy the " L3 robot " by sabotaging its light conversion system , which has been turned into a totem by a primitive tribe . The Doctor and Peri explore a cavern . Peri discovers a sign saying " Marble Arch " — a London Underground sign . This means that they are on Earth . Peri begins to mourn for her planet . The Doctor interrupts the replay to ask what the relevance of this is . He then also asks why Peri is not with him on the station . The Valeyard answers that she is where the Doctor left her , and states that the Doctor ' s evident temporary amnesia — a side - effect of being taken out of time — should soon pass . As the Matrix resumes showing the events on Ravalox , Peri is still upset . The Doctor goes into the complex alone . Two masked figures appear and capture Peri . Meanwhile , Glitz and Dibber are brought before Katryca , Queen of the Tribe of the Free . Glitz claims that the totem attracted the fireball that devastated Ravolox , and asks for it to be taken down . The Queen tells him that others have asked for the totem to be dismantled , and none have succeeded . Glitz and Dibber draw their guns , but are overpowered and locked up . The Doctor finds an underground complex and picks up a bottle of water . This sets off an alarm , and people enter and subdue him . He is accused of stealing water , and sentenced to be stoned . The Doctor tries to block the rocks with his umbrella , but is knocked unconscious . The Valeyard proposes that the inquiry into the Doctor ' s activities should become a full blown trial , with the penalty being the termination of his life . . . Part two . Other officials arrive and break up the stoning . The Doctor is still breathing . Before he can be killed , Merdeen receives a message from the Immortal stating that he wishes to question the Doctor . The Immortal , revealed to be a huge humanoid robot , commands its two assistants to release the service robot . Peri is brought before Katryca , who informs her that as there are few women , she will need to take many husbands as a member of her tribe . She is put in the same prison as Glitz and Dibber . They tell Peri their plan to destroy the robot . They are taken back to Katryca , who tells them that Glitz will be sacrificed because of his attempt to destroy the great totem . The Doctor is taken to the Immortal , who introduces itself as Drathro . It commands the Doctor to work with the two assistants . The Doctor identifies the problem , and tries to leave in order to fix it , but Drathro does not allow him to , as his instructions are to maintain an underground system . The Doctor electrifies the robot and his assistants and escapes . Drathro sends the service robot to track down the Doctor . Meanwhile , Peri , Glitz and Dibber overpower the guards and escape . Dibber remains behind to plant a bomb on the black light converter , whilst they go to the underground complex . In the Marb Station , Merdeen tells Balazar that there has been no fire for hundreds of years , and he should leave the complex . They encounter the Doctor , and Merdeen implores him to help Balazar escape . Peri , Glitz and Dibber , pursued by tribesmen , find the Doctor , and they flee into the Marb Station , but are trapped between the tribe and the service robot . When Peri asks what they should do , the Doctor replies , " I don ' t know . I really think this could be the end . . . " Part three . The Doctor and Peri are saved when the tribesmen shoot at the service robot and disable it . The Doctor tries to re - enter the underground complex , but the tribesmen insist they all return to the village . There , the Doctor is brought before Katryca . She is unimpressed with his explanation of the true nature of the totem and puts them all back in the prison cell . Glitz confirms that the planet is actually Earth . Drathro reactivates the service robot and sends it to the village . It breaks into the building with the Doctor , stuns him after an attempted handshake and takes him away . The tribesmen disable the service robot and decide to attack the Immortal ' s castle to steal his technology , believing that they have killed him . Peri rescues the Doctor from the service robot . They set off to the underground complex to stop Katryca and disable the black light system . The Doctor and Peri encounter Merdeen , one of Drathro ' s train guards , in the corridors of the underground complex . He tells them that he is hunting . When the Doctor asks who his quarry is , he looks at the Doctor and says , " You " . He raises a crossbow weapon at the Doctor and fires . . . Part four . Katryca and the tribesmen arrive at the castle , where they are confronted by Drathro . He electrocutes Katryca and dismisses the rest of the tribe . The Doctor enters Drathro ' s domain , promising to help repair the black light system . However , he determines it to be beyond repair , and tells Drathro that he must shut down the black light system to prevent a massive explosion . Drathro refuses as it would mean its own destruction . The Doctor pleads with him , saying that the explosion could destroy the entire universe . That makes Drathro determined to allow what he thinks is a unique event . Balazar and Peri plead with Merdeen to help them , noting that he would die if the converter exploded . Glitz and Dibber arrive and follow them into the castle through a food chute . Drathro tries to kill them by turning on the food processing system , but Dibber shoots him through the wall . Glitz tells Drathro that they have black light on their ship , and offers to take the robot to the Andromeda Galaxy . Drathro agrees , and leaves with Glitz and Dibber . The Doctor realises that the black light system has already begun to self - destruct . All he can do is prevent it starting a chain reaction . The system explodes , but the blast only destroys the castle , and as a result Drathro collapses . The Doctor and Peri leave Merdeen and Balazar to take the remaining inhabitants to a new life on the surface . The Doctor announces to the court that he has saved the Universe , and starts to present his defence . The Valeyard warns the Doctor that he has more evidence to come , and that the court will demand the Doctor ' s life at the end . Home video and audio releases . VHS releases DVD release DVD Contents