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 would 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.
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 Defense 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 recognizes 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 pulverized dandelion on the grounds. He points out that the force needed to do that would have to be about a quarter ton. He also deduces 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.
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 apologizes 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.
Before K1 can land the killing blow, Sarah (having read the note) arrives. K1 recognizes 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 realized 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 weapons 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 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 defenses. 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.
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 realizes 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 realizes 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 de-materializes 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.