Planet of the Spiders ( TV story ) Planet of the Spiders was the fifth and final story of season 11 of " Doctor Who " . It was the final regular appearance of Jon Pertwee as the Third Doctor and the first of Tom Baker ( uncredited ) as the Fourth Doctor . This was also Richard Franklin ' s final appearance as Mike Yates , outside of cameos . The story was the first to use the term regeneration to describe the biological process that caused Time Lord s to change in physical appearance and introduced one of the Doctor ' s mentors , K ' anpo Rimpoche , as a means to further explain the mechanic and cement it into the " Doctor Who " mythos . Counting K ' anpo ' s regeneration along with the Third Doctor ' s , this is one of two of the Doctor ' s regeneration stories ( the other being " Twice Upon a Time " ) to feature a case of double regenerations and the only televised story where more than one Time Lord regenerates . Synopsis . The blue crystal that the Doctor took from Metebelis III in a previous adventure is desperately sought by the Eight Legs , a race of mutated spiders , as the final element in their plan for universal domination . With help from an old mentor , the Doctor realises the only way to foil the plot is to make the ultimate sacrifice . The Doctor must risk death to return to the cave of the Great One and save the universe . Plot . Part one . In the English countryside , Mike Yates , discharged from UNIT and recuperating at a rural Buddhist meditation centre , is concerned about a small group who meet secretly in the basement . He suspects they may be attempting to summon something evil through their meditation . Meanwhile , the Doctor is researching psychic energy and enlists the assistance of a clairvoyant , Professor Herbert Clegg . His experiments backfire when Clegg ' s fixation on the Metebelis crystal ( which Jo Grant had recently returned to the Doctor ) leads to his seeing giant spiders , and he dies of a fright - induced heart attack . Mike enlists the assistance of Sarah Jane , whose initial scepticism is erased after they are nearly run off the road by a hallucinatory tractor . Sneaking into the basement , they see the group , led by Lupton , summon a giant spider into existence . Part two . The giant spider melds with Lupton , and together they infiltrate UNIT and steal the blue crystal , leading to a lengthy chase involving the Whomobile , Bessie , a small helicopter , a hovercraft , and a speedboat . The Doctor finally catches up to Lupton ' s boat , but when he jumps on board , Lupton vanishes . Part three . Lupton re - appears at the meditation centre , secretly observed by the simple - minded handyman Tommy , who later steals the crystal to add to his collection of " pretties " . The Doctor and Sarah Jane investigate the meditation centre . Exploring the basement , Sarah steps on Lupton ' s mandala and is transported to Metebelis III , where a small group of humans are held in bondage by the giant spiders , who refer to themselves as the Eight Legs . The Doctor follows in his TARDIS to rescue Sarah Jane but is incapacitated by the Eight Legs ' human guards . Part four . On Metebelis III , the Doctor lingers in a coma . Tommy ' s brain is healed when he looks deep into the crystal . Lupton becomes embroiled in a plot led by his spider host against the Eight Leg Queen . The Doctor recovers , thanks to a machine Sarah retrieved for him from the TARDIS . In doing so , however , she is captured by the Eight Legs . Thrown into a cell with a man named Sabor , Sarah learns the history of Metebilis III : the humans are descendants of survivors of a crashed spaceship , and the Eight Legs were common spiders onboard that became mutated by radiation from the blue crystals . The Doctor finds a type of stone which nullifies the effects of the Eight Legs ' energy rays , which the humans will use in their rebellion . The Doctor attempts to rescue Sarah Jane but is captured by servants of the Eight Legs . Part five . The Doctor escapes from his cocoon and reaches the mouth of the cave of the Great One , who warns him not to enter due to the lethal radiation within . She orders him to return the blue crystal that he stole and humiliates him by forcing him to march in a circle like a tin soldier via her psychic power . The Doctor is too overcome by fear to face her and escapes with Sarah Jane back to Earth in the TARDIS . They meet the leader of the meditation centre , the Abbott K ' anpo Rimpoche . Meanwhile Lupton ' s followers , possessed by the Eight Legs , fire telekinetic force at Tommy , the only person standing in the way of their taking the crystal . Part six . Sarah Jane is revealed to be under the control of the Queen , which explains how she and the Doctor escaped so easily . With the blue crystal , which Tommy had given to K ' anpo , the Doctor is able to free her mind . The Doctor is astounded to discover an old friend ; K ' anpo is his former mentor , a fellow renegade Time Lord now enjoying peaceful life on Earth . The attack of Lupton ' s followers is repulsed with the help of Tommy , but K ' anpo is injured and forced to regenerate . He takes on the appearance of Cho Je , who was a projection of K ' anpo ' s inner self all along . The Doctor realises his greed for knowledge and his theft of the crystal has set all these events into motion and that he must face his fear and probable death by returning it . He pilots the TARDIS to Metebelis III and enters the cave to confront the Great One , an enormous spider , who desires the blue crystal to complete a crystalline web that will amplify her psychic power to infinity . The Doctor obliges , but not before warning her that she is doomed to failure . With the crystal in place , she briefly succeeds , before the web overloads and destroys both her and the Eight Legs . The Doctor receives a lethal dose of radiation from the cave and barely manages to escape to the TARDIS . Three weeks later , Sarah Jane and Brigadier Lethbridge - Stewart meet inside UNIT HQ and discuss the Doctor ' s disappearance . While the Brigadier tries to remain optimistic and tells her that it isn ' t the first time the Doctor has been gone for a prolonged period of time , Sarah Jane fears that they might never seem him again . As they speak , the TARDIS appears in front of them . The Doctor stumbles out , a weary look on his face . He manages a warm , albeit weak , smile as Sarah runs up to greet him . He explains to her that he got lost in the Time Vortex ; the TARDIS brought him home . He tries to take a step forward but promptly collapses onto the floor . The Brigadier and Sarah quickly run to his side . Sarah asks him why he had to go back to Metebilis III . He admits that he went back because he had to face his fear ; it was more important for him to do so than to just go on living . As Sarah tearfully begs him not to die , the Doctor cups his hand up to her cheek and softly says , " A tear , Sarah Jane ? Now , don ' t cry . While there ' s life , there ' s . . . " The Doctor cannot finish the quote before he expires . His friends begins to fear that he might truly be dead . As Sarah closes his eyes and weeps , a projected image of K ' anpo appears and assures Sarah Jane and the Brigadier that the Doctor is not dead but still alive , and that he will regenerate and become a new man . They ask when the process will begin ; K ' anpo replies that he is happy to help and gives the Doctor a little " push " to start the regeneration . He then vanishes , telling the Doctor ' s companions , " Look after him . " The Doctor begins to regenerate , much to the surprise of Sarah Jane and the incredulous Brigadier . Sarah Jane exclaims , " Look , Brigadier , look ! I think it ' s starting ! " The Brigadier sighs , " Well . . . Here we go again . " The face of the Third Doctor fades away and reforms . A younger man , with long , curly brown hair , appears — the Fourth Doctor . Home video and audio releases . DVD release . This story was released as " Doctor Who : Planet of the Spiders " . Released : Special Features : Digital releases . This story is available : Video release . This story was released on VHS as per follows :