Robot ( TV story ) Robot was the first story of season 12 of " Doctor Who " . It had numerous notable firsts and lasts . In terms of firsts , " Robot " was Tom Baker ' s debut episode as the Fourth Doctor , which followed on directly from the end of " Planet of the Spiders " . It was also the first story to feature new companion Harry Sullivan , played by Ian Marter . It was the first story for which Robert Holmes was script editor . In terms of lasts , this was the final story produced by Barry Letts . It was the last story in which Nicholas Courtney had a regular role as Brigadier Lethbridge - Stewart , as the character did not appear again until " Terror of the Zygons " the following season . After that , he was entirely absent from the series until " Mawdryn Undead " in 1983 . Likewise , John Levene ' s newly promoted Warrant Officer John Benton made his final appearance as a regular character . His only subsequent appearances were in the aforementioned " Terror of the Zygons " and " The Android Invasion " . Synopsis . Trouble is brewing once again ; top secret plans are being stolen with their guards killed . All evidence seems to point to the culprit being a sentient robot created by a think tank ; however , his basic programming prevents him from killing , providing a contradiction to the clues . At the same time , the Doctor is recovering from his latest regeneration ; can he regain his senses and help UNIT solve the case before time runs out ? Plot . Part one . Sarah Jane Smith and the Brigadier watch as the Third Doctor ' s features fade into those of the Fourth Doctor . The newly regenerated Doctor is delirious , waking to spout random lines from past adventures before again falling unconscious . The Brigadier summons the base medical officer , Lieutenant Harry Sullivan , to take the Doctor to sickbay and care for him . Meanwhile , something large and mechanical enters a Ministry of Defence advanced research centre . It kills a guard and breaks easily through a gate to steal documents from a vault . In the Doctor ' s laboratory , the Brigadier confides to Sarah that the plans are for a disintegrator gun . Sarah asks the Brigadier to arrange for a visitor ' s pass to the National Institute for Advanced Scientific Research , or " Think Tank " , which admits journalists only rarely . The Brigadier is glad to help . The two leave for his office just as the Doctor sneaks into the laboratory . Finding the TARDIS locked , he discovers the key in one of his boots . Before he can enter his ship , he is interrupted by Harry , who tries to persuade him to return to sickbay . When Sarah and the Brigadier return to the lab , they find Harry shut up in a locker and hear the wheezing sound of the TARDIS starting to take off . Sarah bangs frantically on the police box , causing the Doctor to pop his head out . Sarah tries to coax the still unstable Doctor out of the TARDIS , saying they need his help to find a stolen secret weapon . She almost fails , until the Doctor recognises the Brigadier and her . The Doctor has entered the TARDIS to find a new wardrobe . He emerges in a Viking outfit , much to the Brigadier ' s annoyance . The Doctor tries again , but comes out wearing a royal outfit , which he immediately goes back in to change . His third attempt is a clown outfit , complete with face - paint , which is also met with disapproval . His fourth attempt has him in a long scarf and hat , but the Brigadier decides this look is better than the alternatives . The mechanical creature breaks into another facility , this time stealing components from a vault . The Brigadier brings the Doctor to investigate the break - in , but the Doctor seems more interested in a pulverised dandelion on the grounds . He points out that the force needed to do that would have to be about a quarter ton . He also deduce s from the stolen items that whoever is doing this is stealing the components of a disintegrator gun . The last component is a focusing generator . The Brigadier orders the factory where it is housed to be guarded . Sarah visits Think Tank . She is shown around the grounds by the director , Hilda Winters , and her assistant , Arnold Jellicoe . Think Tank is a pure research facility — once research reaches a certain stage , it is handed to another organisation with more resources , like the government . The initial work on the disintegrator gun was done by Think Tank . Sarah notices a door marked " No Admittance " and pushes her way through into the former robotics section run by Professor J . P . Kettlewell . She remembers that Kettlewell left Think Tank after he turned against conventional science . He now works on alternative energy facilities . Sarah nearly slips on a wet patch as she looks around . Jellicoe insists , however , that there is nothing in the section . Winters and he escort Sarah out . At Emmett ' s Electronics , UNIT sets up barriers . The Doctor observes they have covered all directions . . . except down . Sure enough , the mechanical thing tunnels upward into the vault where the generator is kept . By the time the others arrive , all that is left is a dead guard and a hole in the floor . At the other end of the tunnel in the woods , Benton discovers a giant , rectangular and very deep footprint . Sarah goes to see Kettlewell , who brusquely tells her that Think Tank cannot be carrying on his robotics research because no one else has the ability to do so . Still suspicious , she drives back to Think Tank and sneaks back into the robotics section , discovering the wet patch was actually oil . At that moment , a set of doors opens , and a giant , gleaming robot lumbers menacingly towards her , demanding to know who she is and what she is doing here . Part two . Sarah , panicked , runs to where she came in and meets Winters . The director explains that since Sarah was so insistent on seeing what was in there , they activated the robot as a joke . In response to Sarah ' s questions , the robot identifies itself as Experimental Prototype Robot K1 . Its purpose is to replace human beings in carrying out hazardous activities like mining or handling radioactive materials . Sarah asks if the robot can be dangerous . Winters demonstrates by ordering K1 to kill her . K1 is unable to do so and the conflicting impulses cause it distress . Jellicoe explains that K1 ' s prime directive is to serve and not harm humanity . Sarah observes that that was a cruel demonstration and apologises to K1 for its distress , despite Winters claiming that the robot has no feelings . After she leaves , Jellicoe tells Winters that the demonstration was dangerous . K1 ' s programming had just been reset . Jellicoe and Winters adjust K1 again before sending it out . They show K1 a picture of Cabinet Minister Joseph Chambers , telling it that Chambers is an enemy of humanity . Meanwhile , Sarah reports what she has seen to UNIT . The Brigadier cannot act without more evidence . They decide to send Harry to Think Tank , undercover , to gather information . The others go to see Kettlewell , who is hostile at first but warms up when the Doctor shows interest in his work . Kettlewell tells them that he reluctantly ordered K1 dismantled because its capacity to learn and its power began to frighten him . He scoffs at the idea that Winters or Jellicoe would have the ability to alter the robot ' s programming but concedes that , if they had , it would drive the robot mad . K1 enters Chambers ' home , kills him and disintegrates the door of his safe to steal a set of documents . At UNIT , the Brigadier has discovered that many Think Tank scientists , including Winters and Jellicoe , belong to the Scientific Reform Society , a fringe group advocating a society ruled by a scientific elite . K1 arrives at Kettlewell ' s lab in a confused state , knowing it has gone against its programming . It asks its creator for help . When the Doctor and the Brigadier go to Think Tank , Winters tells them K1 has been dismantled , but Winters is aware the Doctor knows she is lying . Meanwhile , Harry arrives at Think Tank disguised as a medical inspector . In his lab , the Doctor receives a call from Kettlewell , who tells him of K1 ' s presence . The Doctor agrees to go over but leaves a note tacked up on the TARDIS in case it is a trap . Indeed , after Kettlewell hangs up , Winters and Jellicoe enter the professor ' s lab . When the Doctor arrives , he finds the place empty except for K1 , who has been ordered to kill the Doctor as an enemy of humanity . Although the Doctor tries to get away , it knocks him out and raises its arm to deliver the coup de grace . Part three . Before K1 can land the killing blow , Sarah ( having read the note ) arrives . K1 recognises her as the person who showed concern for its well - being . When Sarah tells it that Think Tank are deceiving it , K1 flails about in confusion and distress . At that moment , Benton and a squad of UNIT soldiers arrive , but it easily shrugs off their gunfire and escapes . They find Kettlewell tied up inside a cupboard and take him back to UNIT headquarters . There , he confirms that Jellicoe and Winters altered the robot ' s programming and made it unstable . He explains that K1 is made of a living metal he invented , one that can grow like a living organism . That led him to another discovery , a virus that could biodegrade metal into a recyclable form . When Sarah discovers that Kettlewell is still a member of the Scientific Reform Society , she persuades him to attend that evening ' s meeting and let her in secretly . When Benton protests , she points out that neither of them are under UNIT ' s jurisdiction , and they leave . When the Doctor wakes up , he has realised that Chambers must have had access to some kind of ultimate threat . The Brigadier explains that some months before , to ensure peace , the governments of Russia , China and America decided to give the locations and launch codes of their nuclear weapon s to a neutral country , Britain , for safekeeping , with the intention that Britain could publish these codes if war was imminent and allow things to cool down . Chambers was holding on to these destructor codes . If Think Tank has them , they could hold the world to ransom . When Benton tells them that Sarah has gone off with Kettlewell , the Doctor is alarmed . At the SRS meeting , Kettlewell opens a side door to let Sarah in . She hides in the meeting room as Winters addresses the membership , ranting that soon they will rule as is their right . To Sarah ' s shock , she credits this to one man : Kettlewell , who joins them on stage , as does K1 . The robot senses Sarah ' s presence and homes in on her , just as the Doctor arrives to provide a distraction . He knew Kettlewell was the only one capable of altering K1 ' s programming ; the attack on him was a ruse to gain UNIT ' s confidence . Kettlewell explains that for years he had tried to persuade people to stop destroying the environment . Now with Think Tank , he can " make " them stop . The Doctor points out that in science , as in morality , the ends never justify the means , and Kettlewell gets an inkling of Winters ' ruthlessness when she orders the Doctor and Sarah killed . UNIT arrives at that moment , but using K1 as cover and Sarah as a hostage , Winters , Jellicoe and Kettlewell make their escape . Harry contacts the Brigadier from Think Tank , reporting that the scientists are evacuating to a bunker , but is knocked unconscious by Jellicoe and Phillips and taken captive before he can relate any more . UNIT troops proceed to Think Tank ' s atomic shelter but are held off by its automated defences . Winters contacts the Brigadier ; they have already given the governments of the world their demands . Unless these are agreed to , in full , in thirty minutes , she will use the destructor codes . Benton and his men knock out the bunker ' s machine gun nests while the Doctor uses his sonic screwdriver to detonate the mines surrounding the entrance . Winters sends K1 out , armed with the disintegrator gun , which he proceeds to use on a soldier , then a UNIT tank . It warns that it will destroy them all . Part four . In the bunker , Winters has Kettlewell make the international computer linkups so they can use the destructor codes . Kettlewell still believes Winters is bluffing , but she disabuses him of the notion . If the world governments do not give in , she will fire the missiles . The countdown begins . Kettlewell has a change of heart and tries to stop the countdown , but Jellicoe pulls a gun on him . Harry and Sarah have escaped their bonds , and Harry knocks Jellicoe to the ground . Kettlewell holds the countdown while the other two open the doors . As they exit the bunker , K1 swings the disintegrator gun on them . Sarah tries to convince K1 that Think Tank are evil . K1 struggles with the dilemma and fires the gun , disintegrating Kettlewell . With a wail , the robot cries out that it has killed its creator and collapses . With K1 apparently disabled , the Doctor and the UNIT troops make their way into the bunker . Winters has resumed the countdown . Although she moves away from the console when ordered , she is confident that no one can stop the countdown . She has not reckoned on the Doctor , who reprograms the computer and cancels the order . In the mop - up , nobody notices that K1 has revived and has taken Sarah with it into the bunker . The Doctor realises that K1 is in a state of emotional shock after killing its " father " and has developed an Oedipus complex . K1 intends to carry out Kettlewell ' s last orders and ensure the destruction of humanity , although the robot assures Sarah that she alone will be saved . Benton tells the Doctor of Kettlewell ' s description of the robot ' s living metal and the accompanying virus he developed . The Doctor is delighted and tells the Brigadier to find the robot while Harry and he go to Kettlewell ' s lab to cook up a batch of the virus . K1 locks the bunker and restarts the countdown . This time , the world governments ' fail - safe procedures are activated in time . The missiles remain unfired . As K1 exits the bunker , the Brigadier fires at it with the disintegrator gun . Instead of being destroyed , the robot grows to gigantic size . It picks up Sarah like a doll and heads towards the nearby village . It places her on a rooftop as a pitched battle takes place between K1 and UNIT troops , while the Doctor races back with the virus . Driving by K1 ' s feet in his roadster , Bessie , the Doctor throws the batch of virus at it . The virus instantly spreads over the robot , throwing its growth into reverse . It shrinks to doll size and then dissolves completely . Back at the Doctor ' s laboratory , Sarah is saddened by K1 ' s demise . She realises the Doctor had to do what he did , but it had seemed so human . The Doctor observes it was capable of great good as well as great evil , so one could say that it was human . He suggests a trip in the TARDIS to cheer Sarah up . She agrees but wonders why the Doctor wants to do so right now . He has been invited to dinner at Buckingham Palace . Unlike his predecessor , the Doctor finds much of his work at UNIT to be boring . He simply doesn ' t wish to do so much work . Harry enters , asking where they are going . When the Doctor tells him , Harry considers the idea that a police box can go anywhere absurd . The Doctor invites Harry to step inside too , just to prove it is no illusion . Harry steps into the TARDIS and is heard to exclaim in surprise . The Doctor and Sarah follow , grinning . The TARDIS dematerialises just as the Brigadier comes in . Seeing the empty corner of the lab , the Brigadier muses to himself that he will have to tell Buckingham Palace that the Doctor will be a little late for dinner . Home video and audio releases . DVD releases . This story was released as " Doctor Who : Robot " . Released : Notes : Contents : Digital releases . This story is available : Video releases . This story was released as " Doctor Who : Robot " . Released :