"Well, here we go again," the Brigadier remarks as he and Sarah Jane Smith view the white haired third Doctor, on his back on the floor, regenerate into the Fourth Doctor. The Brig calls for Harry Sullivan, a doctor. The Doctor murmurs on the floor after regenerating, "Sontarans perverting the course of human history...a brontosaurus is large and placid and stupid." Sarah reflects to the Brig that is something that happened when they first met. Harry walks in on the word ‘Stupid’. Sergeant John Benton also comes into the room and is told by the Brig about the Doctor changing.

At the Ministry of Weaponry Research Center, something big with metal claws breaks through a fence and patrol dogs, breaks into a steel safe, and steals the research plans for a disintegrator gun.

The Brig tells Sarah about the theft but their talk soon turns to the Doctor. "He used to drive me mad," the Brig exclaims, "...but I miss having him about." Sarah is sure the Doctor will be all right and asks the Brig about getting a visitor's pass into Think Tank's Scientific Research Base for the Frontiers of Science group. They go to his office to do it, Sarah calling the Brig a swinger after he admits he is a bit old fashioned. Sarah thinks of this new Naval Doctor Harry Sullivan is a bit old fashioned. The Doctor hides from them and goes into the lab, wide eyed with joy when he sees the TARDIS but finds it locked. He finds the key in his boot -- an obvious place, he says to himself. Harry comes in and claims, "I am the Doctor" but the Doctor tells him, "I am the definite article." The Doctor proves he is in fit shape by breaking a brick and running in place. Harry checks and finds two hearts, "That can't be right." The Doctor looks into a mirror. "And as for the physiognomy... well, nothing's perfect." He thinks he needs just a bit of time to settle in like a wine. He thinks the nose is a definite improvement and asks Harry about his ears but gets testy when Harry can't really answer. The Doctor thinks Harry has too much time and other things to do -- he doesn't care about his ears! The Doctor tells him he must be off, "Things to do, places to go." Harry tries to stop him but the Doctor grabs up a jump rope and snaps it at him, but then starts jumping rope. Harry jumps with him as the Doctor sings, "Mother, mother, I feel sick, send for the doctor, quick, quick, quick..."

When Sarah and the Brig enter later on, they find Harry tied up and on his back in the cupboard locker and the TARDIS leaving. Sarah knocks on the door and calls the Doctor. The Doctor sticks his face out but slams the door on her, telling her he can go if he wants. He opens it again and Sarah tells him about the Brig's secret break in and adds that the Brig needs his help, his still being UNIT's scientific advisor, he can't leave them in the lurch. The Brig unties Harry, finally. After more door slamming, the Doctor comes out and asks if the Brig is Alexander the Great or Hannibal but finally gets the name right...Brigadier Alistair Gordon Lethbridge-Stewart. He shakes his hand and that of Sarah Jane Smith.

In broad daylight the thing with metal claws attacks another site, disables an electric fence, and chokes a guard to death, breaking the phone off the wall.

Later, the Brig re-enters the lab and the Doctor comes out in a Viking uniform and with a sword. "You've changed!" The Doctor gapes, "Oh no! Not again." After explaining himself and that they need to be discreet as part of UNIT, the Doctor tries again, coming out of the TARDIS in a few moments dressed as a royal subject (of a Queen of Hearts it would seem); then tries again dressed a white clown harlequin with a sad face and tear drops drawn under his eyes. Finally, he emerges with a floppy brimmed hat, a long multi colored scarf and a brown jacket. Then he begins to exit, heading for the attacked site and tells the Brig to explain on the way, "You must cultivate a sense of urgency!"

At the site, the Brig feels the Doctor isn't even following him or paying attention but the Doctor has made several discoveries, blowing dandelion crushed into powder onto the Brig's face and chest. The Brig is respectful of his respect for ecology but...the Doctor tells him that something weighing a quarter of a ton stepped here. He is wearing an eye glass to inspect the ground. The Doctor tells the Brig that the parts that have been stolen are to a compact disintegrator gun. The Brig is shocked the Doctor knows about that. The Doctor puts his finger to the side of his nose.

Sarah visits Think Tank and is greeted by Miss Hilda Winters and Jellicoe, Sarah mistaking the man for the director of the project. Winters is and tells Sarah she didn't expect male chauvinism from her.

The Doctor lays in the jeep, he, the Brig and Harry are travelling in and tells them that whatever it is they are investigating that committed the break ins...is probably not even human, is intelligent since it takes only what it needs. The third ingredient needed for the gun is the focusing generator; the next target. The Brig calls Benton and makes plans to guard Emmett Electronics in Essex, a firm that builds the focusing generator. The Doctor drives Harry and the Brig in the jeep toward the spot.

Sarah is told the Frontiers of Science do the beginnings research on projects, then hand the designs over to others, usually the government. Sarah asks about the disintegrator gun, then barges inside a building where she sees a sign on a door -- Professor J (Jeremiah) P Kettlewell, a man into robotics who left under lots of publicity, to pursue alternate energy sources and sciences. Despite protests from the two of the group, Sarah continues and goes into a storeroom, work area. Sarah smells oil and trips.

A helicopter patrols the electronics firm's underground vault, which is surrounded by UNIT men. The generator parts are in a small metal casket. The place is impregnable, which the Doctor comments is a word like unsinkable. Harry asks what's wrong with the word unsinkable. The Doctor says, sinking down and down, "Nothing said the iceberg to the Titanic...glub, glub, glub..."

The Robot comes up from the floor of the vault and the sentry opens the doors and shoots at it. The Brig, Harry, and the Doctor (losing his hat) race from the jeep to the doors. Harry finds the sentry dead. The Brig finds a large hole. The Doctor mentions a large rat and that the Brig should employ a large cat to get it.

Sarah goes to Professor Kettlewell's house and begins asking probing questions about robotics and why he left. The Professor, with thick eye glasses, and wild hair, puts her off, telling her good day twice. Her visit has unnerved him and he nearly drops his glasses and watches through the window as Sarah leaves.

Benton has found the other end of the hole that the thing dug to tunnel into the electronics vault. Whatever went through it, didn't need to breathe. The Doctor finds very large footsteps.

Sarah manages to flee out a side door to Hilda Winters and Jellicoe, who explain it was just a joke of theirs to prove to her the Robot does work.

The Doctor lays with his hat on his face, talking to the Brigadier about the circumstances of the break ins. The Brig asks if it is alien invasion again. The Doctor rules that out as well as ruling out enemy agents of a foreign power. He believes the threat to be homegrown and asks where Sarah is.

Sarah is being shown the Robot in action by Winters. It is voice controlled and talks to them. It has insufficient data to answer some of Sarah's questions but explains it is Prototype K1, to be used for dangerous tasks such as mining and handling radioactive material. Jellicoe excuses Miss Smith's return as the instincts of a good journalist. When Sarah asks if the Robot is dangerous, Winters tells it, "The girl is an intruder--a spy. Destroy her." It is confused -- it cannot obey since this order conflicts with its prime directive which is to serve and never harm humanity. Sarah calls what Winters did cruel. She believes it has a brain, can walk and talk like humans and might be able to feel. She asks it if it is all right and is sorry for it. She touches it and Miss Winters tells her this is absurd -- that Miss Smith must be the type of girl who gives her motorcars pets names. Winters deactivates it and threatens Sarah's reputation if she should tell what happened here. As Jellicoe moves to show her out, she retorts, "Oh please, don't bother to see me out." After she leaves, Jellicoe tells Winters it was a dangerous thing telling it to kill her. Winters liked it as a test and if it did it, they would have been rid of one problem -- Sarah Jane.

The Doctor is sitting and talking with the Brig and is about to say, "An enormous robot over seven feet tall," when Sarah bursts into the lab and says it instead.

Jellicoe has the Robot head off and working on it. He found it on the last mission when it did not return -- found it near Kettlewell's place. Winters jokes that perhaps Miss Smith is right, perhaps it does have feelings, and misses daddy. She shows the changed Robot a photo of man named Joseph Chambers, telling the Robot this man is an enemy of the human race.

Sarah finishes telling the Brig and the Doctor about Think Tank, suggesting they raid it. The Brig can't start getting the papers for it without creating a fuss that would scare them into hiding the Robot--he needs more information. Harry suggests an inside man; the Doctor suggests a medical doctor -- Harry. Sarah tells Harry it would his chance to be a real James Bond. The Doctor is making a tower of mechanical parts... which all falls down.

The Doctor, the Brig, and Sarah go to Kettlewell's home where the Doctor gets his attention by discussing his solar battery problem, mentioning one half the output is being lost in the error in the 3rd part of his calculations. Kettlewell wants to make an endless supply of power at less cost for mankind. They talk of Galileo and Copernicus and how humans often can't see the answer even when it is under their nose. Finally, the Doctor gets him to discuss the Robot. He ordered them to dismantle the Robot and felt as if it were killing his own son. He has programmed into it his own ideals. It has been tampered with and if its mind is in conflict with its prime directive, it would destroy the mind.

The Robot disintegrates a wall and breaks into the home of Joseph Chambers, killing him with its claws. He was a cabinet minister of security which the Brig later tells the Doctor. The Robot has also used the disintegrator gun on the man's safe and rob it.

The Brig has run a security check on Think Tank and the members seem an exemplary lot. Many of them, though, are member of the Scientific Reform Society, an organization formed years before to reorganize the world along scientific and rational lines. There has been a rush of new members, young members, some computer technicians and well known scientists. Sarah tells them she is going home to bed--she still has to work, parting with the fact that even crank reform groups need free publicity. The Brig asks if they are going to let Miss Smith do all the work. The Doctor is laying down on a table, tells him tomorrow they shall visit Think Tank, and shuts the light off to sleep.

The Robot goes to Kettlewell's home and reveals his conflicting orders. He has been programmed not to kill but has been ordered to by Winters and Jellicoe.

Sarah sees Mr. Short of the Scientific Reform Society who has a large bodyguard hovering menacingly nearby. He tells her the Society believes that superior types should rule and guide and help inferior types, keep them away from ideas that may be dangerous to all. He comments on Sarah's trousers as one such idea. There is to be a meeting tonight and when Sarah asks if she can attend, Short doubts she is the right type to join and is barred. He still wants good publicity of the group. Sarah makes a sarcastic comment about it being mentioned between the flying saucer groups and the Flat Earthers. Short picks up a phone when she leaves.

The Doctor is being shown around Think Tank that morning by Winters. He calls it an amusing visit and tells her that scientific research has to start somewhere. He asks to see the Robot and barges into the building. Winters tells him it has to be destroyed in the furnace since Miss Smith introduced it to useless ideas. The Doctor counters with sarcasm, "...you mean concern, compassion, useless things like that?" When the Brig wants to get the authority to search, Winters gives him the right to search now. The Doctor tells them, in that case, they need not bother, "C'mon along, Brigadier." Phillips will show them back to their car. At this time, the Ministry of Health has sent someone over to check all the medical records of their staff Harry Sullivan.

Again, the Doctor and the Brig talk. The Brig will go see if he can get the Minister to approve a raid on Think Tank. The Brig comments before he leaves that the Doctor isn't listening to him and he believes it is due to the Doctor engaging in more thinking. After he leaves, the Doctor snaps out of his wide eyed thinking, "I beg your pardon, Brigadier, I was just thinking." The phone rings and the Doctor tells the UNIT operator, "I'll talk to anybody." It is Kettlewell who tells him about the Robot coming to him. They will meet at Kettlewell's home. After hanging up, Kettlewell opens the door and Winters and Jellicoe are there, menacing him.

The Doctor super speed types a note to Sarah and the Brig and leaves it taped to the TARDIS -- telling about his trip to Kettlewell's -- he thinks it may be a trap and he can handle it if it is but if it isn't, he has left the note. The Doctor drives off in Bessie.

Before entering the lab, Benton tells Sarah that the Brig should have more men under his command. In that case, the powers that be promoted Benton to Warrant Officer instead of hiring and paying more men. Sarah congratulates him, "About time, too." They go in and Sarah finds the note, "Oh the idiot, he thinks he can cope with anything." Benton rounds up some men, telling Miss Smith to wait for him to do so but she leaves in her car.

Sarah drives up next to Bessie, looks into the wooden door that has been cracked, and races inside, stopping the Robot from killing the downed Doctor. Benton arrives and shoots at it and soldiers fire from the other door. It smashes out of the wooden door and hits a soldier down. Sarah asks Benton why he had to start shooting for. Hearing a sound, Benton opens a cupboard and Kettlewell, tied up, falls out.

Back at UNIT's lab, Kettlewell talks to Sarah and Benton. He goes on about how the Robot is made of living metal which can even grow. Sarah comments, "It's big enough for me right now." Kettlewell also made a metal virus which can get rid of metallic waste of the planet. Benton brings Kettlewell some tea. Kettlewell still has his membership card to the Scientific Reform Movement. Sarah hits upon the plan to get him to smuggle her in to the meeting tonight. Benton starts to stop them, "Hey hang on you two..." Sarah tells him the Doctor needs his rest, Benton should not wake him. Since she and the Professor are not official members of UNIT, he cannot stop them. Telling Benton to go polish his rifle or something, Sarah escorts the Professor out.

Later, the Brig chews out Benton for letting the two of them leave. The Brig didn't get permission to raid Think Tank. The Doctor bashes into the room and tells him then he must do it anyway. Chambers was the guardian of the ultimate threat, the Doctor has worked out and the Brig confirms it. Some months ago, the three super powers found a way to make peace...each of them have hidden atomic missile sites with a neutral country having all three site locations and other details. In the event of trouble, all the details would be published, "Naturally enough, the only country that could be trusted with such a role was Great Britain." The Doctor quips, "Naturally, I mean the rest were all foreigners." The Brig says, "Yes," then realizes the Doctor's comment was sarcastic. The destructor codes for firing the missiles were in Chamber's safe...which the Robot stole. Think Tank can set off every missile in the world now. Benton tells the Doctor about Sarah. The Doctor snaps, "Kettlewell! You let Sarah go off somewhere with Kettlewell!!!?"

Kettlewell gets into the meeting building, then opens the doors for Sarah. As she hides a table of boxes, takes pictures and tape records the meeting, Winters gives a fanatical speech about the elite ruling the world. Sarah is shocked when Winters announces Kettlewell and brings forth the Robot to the stage.

The Doctor is in the lobby, trying to get in past the guard. He has taken out many things from his pockets including a toy dove, an edict scroll he says is the freedom of the city of Skaro, a telescope, his pilot's license to the Mars-Venus rocket run, his galactic passport ("Do you travel much?" he asks the guard), and his honorary membership card to the Alpha Centauri table tennis club ("Tricky opponents, six arms and six bats, of course.") The guard menaces the Doctor, who starts out but uses the backlash of his scarf to trip the man down. He pats the man, telling him everything is going to be all right, then goes into the room.

The Robot has pushed Kettlewell aside and knocked the boxes off the table, revealing Sarah Jane. Winters calls her a spy and tells the Robot to kill her. The Doctor enters and grabs the microphone, asking if the audience wants song and dance, whips out some cards and asks for a volunteer from the audience. Winters directs the guards to get him but as one dives at the Doctor, he ducks and the guard falls over him. The Doctor helps him up but his arm is twisted and two of the men hold him, "Any other sporting member of the audience...?" The Doctor asks Kettlewell why. He cannot make the world see that they are spoiling the planet; now with his new friends, he can. The Doctor tells him that, science as in morality, the end never justifies the means." Winters tells Kettlewell the two will have to be killed. He just wants to lock them up.

The Brig arrives and starts shooting. The men let the Doctor go. The Doctor falls onto the table. Winters grabs Sarah and heads out; flanked by Kettlewell and Jellicoe, who are protected by the Robot, which they order to. Benton and the Brig cannot get in a shot past it. Past barb wire, Winters drags Sarah to a large truck; Jellicoe, Kettlewell, and the Robot get in the back and the truck takes off through the downed truck stop.

The Doctor tells the Brig that only Kettlewell could reprogram the Robot; Kettlewell was not hit in the head at his place. Harry Sullivan calls, telling them the research center is being evacuated and that the group is heading to a bunker. Before he can tell more, he is hit and knocked out by two of the men.

The Brig surmises the bunker used by Think Tank members during the Cold War is the site the group is headed. UNIT goes to the bunker but the Robot, Kettlewell, Winters, Sarah, Jellicoe, and others are already there. Just in time, the Doctor sees body-heat activated guns in turrets and he warns the Brig and his men to get down. When the radio comes on, the Doctor covers Benton's mouth but Benton tells him it is the radio. He goes to the jeep and it is Miss Winters who wants to talk to the Brigadier. The governments have their demands. If not met, the missiles will be fired. The "wretched woman" as the Brig calls her to the others after their conversation, tells them they have associates all over the world. She gives the Brig 30 minutes to surrender. The Brig orders Benton to use grenades to knock out the guns.

Sarah is brought into a storage room with Harry and the Robot is standing guard, threatening Harry. Sarah is tied up just as Harry is and sat on the disintegrator gun box. She mocks Harry, "James Bond."

The guns are blown up from outside but the Doctor stops them yet again. He uses the sonic screwdriver to blow off the minefield that is between them and the bunker doors. At the doors, the Doctor stops Benton from getting more explosives ("I think we've had enough bangs for one day, aye, Brigadier?") and changes the sonic screwdriver into a miniature lance to cut out the lock on the huge doors. Jellicoe puts the disintegrator gun on the Robot. Sarah tries to talk it into not doing this but it goes out. Somewhat reluctantly, Kettlewell has the destructor codes operational now.

Kettlewell finished the preliminary link up and asks Winters what if the authorities don't surrender and call Think Tank's bluff. Winters tells him then they will fire the missiles, either way they will start a new world order. Kettlewell starts the countdown and it moves to 295. Harry asks Miss Smith how her ropes are coming. Winters and Jellicoe move out of the control room to check the food and water storage in case they have to spend years in the bunker. Jellicoe stops to check on Harry and Sarah, whom Winters says will have to disposed of. Jellicoe points the gun at them but she says, "Later," and the pair of horrible people leave.

Outside in the field, The Doctor tells The Brig that the range of the disintegrator gun is such that it could put a hole in the surface of the moon and its power is unlimited.

Inside the bunker, Kettlewell backs out of it and tries to smash a chair into the computer firing mechanism but Jellicoe, gun in hand, stops him. Harry stops Jellicoe, punching him down as he and Sarah come running into the room. As the Doctor is about to try to sneak around the Robot, knowing he hasn't a chance, Sarah runs out. The Doctor runs to her and they try to convince the Robot to stop. Harry and Kettlewell are there by now as well. Sarah explains to the Robot over and over but it is confused, "I must destroy you!" To save Sarah, Kettlewell tries to stop it and jumps in front of the gun but is vaporized. The Robot goes insane, knowing it killed the one who created it. It falls and as the others race inside, Benton picks up the fallen disintegrator gun. Winters watches the others on a TV camera screen and resets the launching of the missiles using the codes. The Brig comes in and points a gun but Winters tells him he won't shoot. Sarah finds a fallen gun and tells her, "Maybe he won't but I certainly will." She moves out of the way and the Brig, infuriated, screams at her, "Cancel the codes!" But she won't. The Doctor races into the chair, thumbs through the notebook at the console about the computer and claims Winters may be right--they may not be able to stop it. Benton takes Winters and the unconscious Jellicoe out.

As Harry, Sarah, and the Brigadier watch, wide eyed and nervous, they see the Doctor calmly talking excited but unnerved as he explains why computer are sophisticated idiots--tell them to destroy you and they will do it unless... you do what the Doctor does and stop them. The launch stops at 002. Everyone relaxes. Sarah slips off but runs into the awakened Robot.

Later, Benton tells the Brig he has mopped up the rest of that Think Tank group (well, where in the bunker were they?) and missed Miss Smith leaving. He assumed she went home. The Robot is in a state of emotional shock, The Doctor explains -- it can only turn to the one person that ever showed it kindness -- Sarah Jane.

Sarah tells the Robot, "It's all over." The Robot can still bring about the destruction of humanity but she alone will be saved. The Robot gets a signal on itself that the bunker is cleared. He reenters it from another room.

The Brig puffs, "For once I'd like to meet an alien menace that wasn't immune to bullets." Benton explains about Kettlewell's talk to he and Sarah earlier -- about the living metal and the metal eating virus. The Doctor touches his arms, "Wonderful, Mr. Benton, wonderful!" Harry and the Doctor leave in Bessie for Kettlewell's place, leaving a smiling Benton. The Brig looks at Benton and Benton drops the smile and stands at attention.

As the Robot herds Sarah toward the control room, a soldier blocks their way. Sarah gets him to leave without firing his rifle. The Robot lets him go. The soldier tells the Brig where Sarah is. The Robot re-starts the countdown to destruction. Sarah tries to interfere and it hits her down to the floor. It wants to carry out its creator's plan. She tells the Robot that his creator changed his mind -- he wouldn't want the Robot to do this. The Robot tells her that humanity is not worthy to survive. He will build more like him...more machines, "Machines do not lie."

At Kettlewell's lab, the Doctor thumbs through notes and scraps of paper, "Why didn't that silly man write up his experiments properly!?" The Brig calls Harry via the walkie talkie. The Doctor spouts to Harry, "Tell him to stop pestering me!" He needs time to get the metal eating virus right. The Brig tells him the current situation. The Doctor believes it to be part of the Oedipus Complex that the Robot is going through. The Brig cannot turn off the electricity to the bunker. The Doctor warns him that the world powers should be notified immediately -- the crisis is not over and they should activate their fail safe procedures. The countdown is stopped before it can be finished. This infuriates the Robot who claims, "I cannot be destroyed!" It takes her upward.

The Doctor finishes the metal eating virus and tells Harry, who has been passing him tools and things, "We've done it, Harry!"

Sarah and the Robot come out of the double doors of the bunker. The Brig tells his men not to open fire for fear of hitting Miss Smith. She might find a chance to escape. The Brig handles the disintegrator gun, realizing he can use it, "Maybe just for once, we're not going to need the Doctor." He shoots the Robot which makes the Robot grow and grow into an even larger giant, over 50 feet tall or more. Sarah makes a run for it but is scooped up in one of its large claws. It walks through town and eventually places her on the roof of one of the high buildings.

Benton calls Smith and Haskell to fire the rocket launcher at it while the other men toss grenades. In Bessie, the Doctor and Harry speed toward the area. The Robot picks up some cars and throws them. The Brig orders a fall back as it keeps coming. One solider keeps shooting and tries to give the others time or something... and he runs but is stepped on and killed by the Robot. The Robot also kicks over a house!

Bessie pulls up to the area. Harry sees the Robot, "Curiouser and curiouser." The Doctor says, "Said Alice." The Robot walks near an electric power pole and is zapped by the tension wires. It hits the pole over. Bessie pulls up to the Brig and Benton. The Brig sent for the artillery and the RAF (Air Force) are on their way. The Doctor stands up in Bessie, holding a bucket of the virus, bubbling pinkish over the side, "I hope that won't be necessary." He tosses his hat to the Brig, ordering Harry to drive on. The Brig wanted to stop him but tells Benton to go and help Miss Smith down. The Doctor hits the foot of the Robot with the virus which is in an active solution as he and Harry drive past. In obvious pain and suffering, the moaning Robot slowly shrinks down to a smaller size, smaller than a man. The Brig tosses the Doctor's hat to him. The Robot body vanishes completely.

Later, in UNIT lab near the TARDIS, Sarah is sad, thinking, staring into space. The Doctor comes in and offers her a jelly baby from a bag. He tries to cheer her up. He turns from the TARDIS and tells her he had to do it. It was capable of great good and great evil, "You could say it was human." He wants to take a trip in the TARDIS and tells her it is a free cosmos. The Brig wants him to attend a cabinet lunch at Downing Street, have dinner at the Palace, and write 17 reports in triplicate. In anger, and acting like a child, the Doctor rants, "I won't do it I tell you, I won't." He hits his hand on the table and then puts it in his mouth, having hurt it. Sarah laughs, telling him he's being childish. The Doctor comments, "There's no point in being grown up if you can't be childish sometimes." Sarah now takes a jelly baby from him and they move toward the TARDIS door.