Castrovalva ( TV story ) Castrovalva was the first story of Season 19 of " Doctor Who " , and the first full television serial to feature Peter Davison as the Fifth Doctor . Beginning with this story , " Doctor Who " was moved from its traditional Saturday evening slot on BBC1 to air twice weekly . Synopsis . The Doctor ' s latest regeneration has proven more unstable than his previous ones . His two assistants , Tegan and Nyssa , help him recuperate on the tranquil planet of Castrovalva . Adric has been captured by the Master , who is taking advantage of the Doctor ' s weakened state to ensnare him in an elaborate trap whose recursive nature threatens to destroy his mind . As the Doctor begins to recover he realises that the Master ' s trap for Adric is even more intricate than he could have imagined and that he will stop at nothing to gain his revenge over the Doctor . Plot . Part one . The new Doctor collapses as his companions try to carry him back to the TARDIS , and they are all captured by the Pharos Project guards . The unconscious Doctor is loaded into an ambulance while his companions are frisked for weapons . Adric tries to convince the sceptical guards that he and his friends are alien intelligences who came in answer to the Pharos Project ' s call . This distracts them long enough for Tegan and Nyssa to steal the ambulance and drive it to the TARDIS . Once inside , the Doctor regains consciousness and bolts into the corridors . Before Tegan and Nyssa can follow him , the Master ' s TARDIS materialises outside , and bolts of energy strike down Adric and the guards . Tegan and Nyssa emerge from the Doctor ' s TARDIS just as the Master ' s vanishes , leaving a stunned Adric kneeling amongst the unconscious bodies of the guards . Tegan and Nyssa usher him into the TARDIS , where he immediately crosses to the console and sets the controls for flight . Without so much as a word of thanks , Adric then sets off to join the Doctor , who is unravelling his old clothing to leave a trail as he searches his ship for its Zero Room . The Doctor ' s erratic personality is beginning to disintegrate , and he drifts through behavioural patterns of his previous incarnations , calls Adric by the names of former companions - and realises that his regeneration is failing . While waiting for the Doctor and Adric to return , Tegan and Nyssa discover the TARDIS data bank and try to find its index file . The frustrated Tegan eventually points out that if they had an index file they could look up the index file in the index file under " index file " . Nyssa points out that this is a perfect example of the mathematical concept of recursion , and Tegan , musing on the power of the word " if " , realises that " I . F . " stands for " Index File " . They are thus able to access the TARDIS ' destination setting and determine that they are on a programmed course and unlikely to crash . But they have no idea what " Hydrogen Inrush : Event One " means . The oddly unresponsive Adric abandons the Doctor in the corridors and sets off in search of the Zero Room on his own , growing ever more worried and panicky as he does so . The Master , watching from the safety of his own TARDIS , is amused by his behaviour , knowing Adric will never be free until their task is complete . The Doctor , left to his own devices , finds a room filled with cricketing equipment and changes into a cricket outfit with a beige coloured trenchcoat and a panama hat which is to be his new attire . He then hears the distant sound of the Zero Room door slamming shut , and rushes off to investigate , running into Tegan and Nyssa on the way . He is continuing to weaken and nearly collapses before Nyssa finds the Zero Room , an environment completely isolated from the rest of the Universe . Once cut off from the complexity of the outside world the Doctor ' s health is restored , although it will take quite some time for him to fully recover from the stress of his latest regeneration . The Doctor drifts off to sleep , telling his companions that they will all have their parts to play in helping him to heal . But Tegan and Nyssa see Adric pinned against a wall of the Zero Room - - and he warns them that he is in fact a Block Transfer Computation and that the real Adric is now a prisoner of the Master . The image of Adric breaks up while warning the girls to check the co - ordinates ; the Master made him set a trap . Nyssa decides to investigate on her own , leaving Tegan to watch over the Doctor while she returns to the console room . On her way back , she notices that the ambient temperature in the corridors is increasing . Meanwhile , the Master muses over the curious fact that the image of Adric had nearly developed a will of its own towards the end . The Doctor awakens , convinced that something is wrong somewhere - and then the cloister bell begins to sound . He insists upon investigating but as soon as he steps out of the Zero Room he falls as though struck . Tegan orders him to remain in the Zero Room while she finds out what ' s going on . The Doctor is unable to remain in the Zero Room and do nothing , but when he emerges the TARDIS begins to buck under some sort of turbulence and throws him unconscious to the floor . Tegan arrives in the console room as the temperature continues to increase , to find that Nyssa has used the data bank to research the " Hydrogen Inrush " - and has discovered that Adric programmed the TARDIS to travel back in Time to the creation of the universe . As the TARDIS begins to shake with turbulence , the Master appears on the TARDIS ' scanner , gloating , as Adric hangs pinned in a web of power lines in the background . . . Part two . Nyssa turns off the scanner screen so she won ' t have to see the Master any longer , and she and Tegan try to find some way of escaping from the time pressure of Event One . The Master , meanwhile , hovers nearby in time and space , using Adric as a source of mathematical calculations to generate Block Transfer Computations . He attempts to visualise the destruction of the Doctor ' s TARDIS , but Adric ' s resistance interferes with the reception . The Master suggests that as the Doctor is doomed , Adric might as well join forces with him and co - operate willingly . . . and after some thought , Adric agrees . The Doctor regains consciousness as turbulence drops medical supplies from a TARDIS roundel onto his head and sends an electric wheelchair rolling down the corridor towards him . He manages to get to the console room , where Nyssa realises that the neurochemical reactions stimulated by the crisis have focussed his concentration . For the moment at least , he is thinking clearly , and he sends Nyssa to vent the TARDIS ' thermal buffers while showing Tegan how to use the Architectural Configuration . If they delete a quarter of the TARDIS ' internal structure they will generate enough power and thrust to escape the time pressure of Event One . But as the temperature inside the TARDIS returns to normal the Doctor ' s body chemistry stabilises - - and he loses consciousness before he can explain how to ensure that the console room is not deleted . Tegan and Nyssa have no choice but to delete a quarter of the TARDIS at random . The Master watches and gloats as the TARDIS is apparently destroyed in the nexus of Event One . He prepares to release Adric from the power web , as he no longer requires his backup plan - - or so he thinks until he finds residual power in the Hadron lines , revealing that Adric is picking up an image and hiding it from him . He boosts power through the web , burning through Adric ' s resistance - and revealing that the TARDIS has survived and escaped Event One after all . Tegan and Nyssa look up information on failed regenerations in the TARDIS data bank , and find a suggestion that Time Lords can relax in Dwellings of Simplicity with little or no technological advancement . The suggested location is Castrovalva , the central habitation of the planet that forms the Andromedan Phylox Series - according to the TARDIS records at this point . Tegan prepares to pilot the TARDIS to Castrovalva while Nyssa returns the Doctor to the Zero Room . The Doctor , sceptical that Tegan can actually pilot the complex time machine , is convinced that someone else is responsible for the journey - although he can ' t think of who might be . But he soon has a greater problem , as Nyssa opens the doors of the Zero Room to reveal that it has been jettisoned . Tegan apparently pilots the TARDIS to Castrovalva and lands in the forest outside , materialising the ship on an angle . Meanwhile , the Doctor has given his sonic screwdriver to Nyssa and told her to take the doors off the wall - and when the ship touches down the door of the Zero Room falls off its loosened hinges and balances against the wall . The Doctor , partially shielded underneath , begins to recover and is finally able to explain that they can build a Zero Cabinet out of the pieces of the Zero Room which remain . Nyssa and Tegan then prepare to carry the Zero Cabinet to Castrovalva , balancing it on the electric wheelchair ; but as the journey progresses they accidentally knock the wheelchair into a creek , shorting out its motor and locking its wheels . Nyssa falls into the creek while retrieving the wheelchair and finds that her ion bonder is now waterlogged . Unable to repair the wheelchair , they are forced to continue their journey on foot - unaware that they are being observed . As evening approaches , Tegan spots Castrovalva in the distance , a walled city atop a tall cliff . Nyssa hides the Zero Cabinet under a covering of leaves , just in case , and she and Tegan set off to look for a way in - unaware that nearby , spear - carrying warriors dressed in feathered suits of armour have observed their arrival and are discussing what to do about it . The Doctor , unaware of the warriors ' presence , opens the Cabinet . Soon afterwards , Nyssa and Tegan return , having failed to find an easy way into the city - only to find blood on the ground , the Cabinet open , and the Doctor missing . . . Part three . Nyssa and Tegan follow the trail of blood away from the Cabinet , trying to find the Doctor while avoiding the feathered warriors in the forest . They eventually spot the Doctor some distance away , but although they call out to him he doesn ' t recognise his own name - - all he knows is that he , too , is looking for someone called " the Doctor " . He follows the trail of blood to a doorway in the cliff , where a group of warriors who have killed a wild pig are waiting for their fellow , Ruther , to arrive . Ruther ' s group arrives carrying the Zero Cabinet , and Ruther finds the Doctor hiding in the bushes . The warriors take the Doctor into Castrovalva with them , and Nyssa and Tegan arrive seconds too late as the door seals itself shut . The Doctor is surprised to find that Castrovalva is a pleasant , villa - like city with an open courtyard , tiled floors , and soft pastel colours in its walls and pillars . Furthermore , the " warriors " are all in fact placid , middle - aged men who are greeted upon their return by Shardovan , the dour town librarian . Castrovalva is in truth a most civilised town , and its inhabitants were merely engaged in an exercise programme and were wearing old suits of armour to stiffen their resolve for the hunt ; Shardovan did not go as he is too tall to fit into the available armour . The Doctor , still uncertain as to his own identity , is taken to the guest rooms to spend the night ; there , he is greeted by the Portreeve , a kind elderly man who oversees the daily life of Castrovalva . The Doctor drinks a medicinal tonic prescribed by the town ' s herbalist , Mergreave , and goes to sleep , assured by the Portreeve that he ' ll soon find the Doctor he seeks . Nyssa and Tegan scale the walls to get into Castrovalva , and are surprised when a rope ladder is lowered down to them . Inside , Shardovan decides not to tell the Portreeve about the new arrivals , and is unpleasantly surprised to find that the Portreeve is already aware of them and has been standing on the walkways above the courtyard all this time . Mergreave shows Tegan and Nyssa that the Doctor is sleeping peacefully and shows them to their own guest quarters . But Adric emerges from the shadows in the Doctor ' s room , and watches them go . . . The next morning , Nyssa leaves Tegan sleeping peacefully and sets off to explore Castrovalva . While passing through the courtyard she encounters a group of Castrovalvans carrying the Zero Cabinet , and redirects them to the Doctor ' s room . There , Adric appears to her , and warns her not to tell the Doctor about his kidnapping ; it ' s important that the Doctor remain in Castrovalva until his regeneration is complete . Adric ' s image then breaks up , as the Doctor awakens , feeling much better . The Master is satisfied ; now , they will remain untroubled by the Doctor ' s meddling . The Doctor , Tegan and Nyssa breakfast with the Portreeve , and then Nyssa and Tegan set off to search the town library for books on telebiogenesis . Unfortunately , the library has no technical section , and they decide to read up on the history of Castrovalva instead . Meanwhile , the Portreeve shows the Doctor the source of his knowledge : an ancient tapestry designed with long - forgotten techniques , which reweaves itself to display events of great importance such as the Doctor ' s arrival . As the Doctor watches the images of Tegan and Nyssa carrying him through the forest he begins to get the impression that someone ' s missing , and a chance encounter with a young girl inspires him to remember Adric . The Doctor returns to his room to confront Tegan and Nyssa , who are forced to admit what has happened . The Doctor insists upon leaving Castrovalva immediately to rescue Adric . They pass through the town square , where the women of Castrovalva are doing their laundry - but find their path leading impossibly back to the square , no matter which direction they take . The Doctor collapses , suffering a relapse as a complex spatial distortion begins to take effect around him . Tegan and Nyssa manage to get him back to his room , only to find that the Zero Cabinet has been removed - and outside the window , the distortion of Castrovalva is much more apparent . The town is deliberately folding in on itself , and the Doctor and his companions are trapped . . . Part four . The Doctor collapses , urging Nyssa and Tegan to find the Zero Cabinet . For the moment , he is able to block out the effect of the spatial anomaly by placing a silver - backed mirror in front of the window . While waiting for his companions to return he leafs through the history of Castrovalva and finds a piece of paper with some very curious implications . Mergreave arrives , and the Doctor questions him about the geography of Castrovalva . He seems not to understand the Doctor ' s confusion until the Doctor has him draw a map of Castrovalva on the back of the mirror - - and he is taken aback to discover that he can locate his single pharmacy in four different locations on the map . Ruther leads Nyssa and Tegan in circles about the town but seems to see nothing amiss . Tegan is convinced that the people of Castrovalva are all involved in a conspiracy , but Nyssa suspects that they are also a part of the mathematical recursion affecting the town . They finally realise that the women in the square are doing their laundry in the Zero Cabinet and demand that it be returned to the Doctor . There , the Doctor tests Ruther , who is also surprised to find that he can find a single location in four places on the map . The Doctor is convinced that he is overlooking something obvious about the history of Castrovalva - and the notes he has found , in Shardovan ' s handwriting , indicate that Shardovan also knows what it is . The Doctor suspects that the books are forgeries , to cover up something about Castrovalva ' s " real " history - if indeed there " is " one . The Doctor requires answers from the Portreeve but must be carried to his home in the Zero Cabinet ; but while Tegan ushers Mergreave and Ruther out of the room , the Doctor suggests an alternate plan to Nyssa . When Tegan and Nyssa emerge from the Doctor ' s rooms with the Cabinet , Tegan rebuffs Shardovan ' s offer to help carry the Cabinet . Shardovan is then lured away from the procession and confronted by the Doctor , who filled the Cabinet with all thirty volumes of the history of Castrovalva . He knows from Shardovan ' s notes and refusal to join the hunt that Shardovan suspects the whole history of Castrovalva is a fiction ; but even the Doctor fails to realise the extent of the fiction until Shardovan points out the obvious fact he has overlooked : the books are five hundred years old , but they chronicle the history of Castrovalva to the present day ! Shardovan takes the Doctor to the Portreeve ' s home , to break in and confirm the Doctor ' s suspicions about the true nature of Castrovalva . The Portreeve dismisses all townspeople but Mergreave and Ruther , who stand by in confusion as the Portreeve sheds his disguise , revealing himself to be the Master . Castrovalva is in reality a Block Transfer Computation , brought into existence and maintained by Adric ' s calculations ; the entry in the TARDIS data bank was planted by Adric to lure the Doctor into the trap . The Master attempts to break into the Zero Cabinet , insanely determined to have one last look at the Doctor ' s face before he destroys him , but the Doctor breaks in and confronts him . The Doctor rips down the ever - changing tapestry to reveal Adric pinned behind it in a web of power lines . The Doctor is powerless to stop the Master as he begins to dispose of the bait , casually causing Ruther to vanish when Ruther tries to attack him . But Shardovan demonstrates his free will by swinging from the balcony on the chandelier into the web , giving his life to break the strands and release Adric . Without the support of Adric ' s constant calculation , the computation that is Castrovalva begins to collapse in upon itself . The Master tries to flee in his TARDIS , which was disguised as the Portreeve ' s fireplace , but space is folding in too fast and his TARDIS is unable to escape from Castrovalva . Mergreave tries to lead the Doctor and his friends to safety but soon even he can no longer find his way about - but Adric , who created Castrovalva , can see clearly and guides his friends out through a breach in space onto the hillside . As the Master rushes for the breach , Mergreave remains to hold him back , and the Doctor is forced to watch helplessly as the breach closes up , trapping the Master along with the doomed Castrovalvans . As Castrovalva disappears permanently , the Doctor and his companions jog back to the TARDIS . There , the Doctor reveals to Tegan ' s disappointment that she did not pilot the TARDIS after all ; the whole flight had been pre - programmed by Adric and the Master ; of the latter , the Doctor says , " The Master leaves nothing to chance . " But she need not worry about piloting the TARDIS away ; the Doctor ' s regeneration has finally concluded , and he cheerfully says that " Whoever I feel like , it ' s absolutely splendid . " Story notes . Production errors . Between " Logopolis " and " Castrovalva " . The first episode of this story is narratively continuous with the last episode of " Logopolis " . Home video and audio releases . DVD releases . This story was released on DVD together with " The Keeper of Traken " and " Logopolis " as part of the " New Beginnings " DVD box set . Released : Contents : Notes : Doctor Who DVD Files . It was released with issue 47 of " Doctor Who DVD Files " . Video releases . This story was released on video in 1992 in the UK and Australia and 1993 in the US .