The Doctor: Emotions. Love. Pride. Hate. Fear. Have you no emotions, sir?
Cyberman: Come to Mondas and you will have no need of emotions. You will become like us.
The one where the Doctor says goodbye... for the first time.
The TARDIS materialises at the South Pole in the far future of 1986, where a manned space probe is being talked down to Earth by a Space Tracking Station. The capsule is in difficulty, the Doctor realises, because of the gravitational pull of another planet that is moving toward Earth.
This planet turns out to be Mondas, Earth's "twin planet" from long ago, and is now the home of the Cybermen, a race of human-like beings who have over many centuries replaced their limbs and organs with metal and plastic. The Cybermen attack the tracking station while Mondas starts to drain Earth's energy. Ben stops General Cutler, in command of the station, from launching a powerful "Z-Bomb" to destroy the Cybermen and Mondas eventually disintegrates from absorbing too much energy. The Cybermen are apparently entirely dependent on their planet, as once it is gone they all collapse and die.
The Doctor is also weakened and hurries back to the TARDIS without Ben and Polly. In a series of events that would remain unrevealed to us for another fifty-one years, upon returning to his TARDIS, his body beginning to glow with strange energy, the Doctor notices a similar TARDIS, with another silver-haired gentleman in a state not unlike his own. After a circumstance involving glass people, a World War I captain, and a future companion of his, he returns to his own TARDIS, where he collapses and the TARDIS takes off of its own accord, Ben and Polly having caught up. Before the humans' astonished eyes, the Doctor's face glows and becomes that of a completely different man...
This was William Hartnell's last story as the incumbent Doctor, his tenure ending at twenty-nine stories, and the debut of Patrick Troughton as the Second Doctor, in the first instance of regeneration on screen. Since this was just an experimental plot element at the time, the actor changeover occurred partway through Season 4, an unusual move that would only be repeated once, eighteen years later. Later regenerations would normally not take place until a respective season finale or end-of-year special to cap off that Doctor's tenure. A few of the Doctor's regenerations have been depicted under unique circumstances, when they were sudden plot twists or "past due", so to speak. Regeneration was not an ability exclusive to the Doctor, as it would be explained as a feature of the Doctor's species and demonstrated by others like him as well.
Episode 4 is, unfortunately, one of the missing ones (although footage of the regeneration itself survives). An animated reconstruction of the fourth episode has been produced for the story's DVD release, initially in June 2013 as part of a Limited Special Collector's Ultimate Edition box set of all the Doctor's final stories, and later on its own. A brief segment was also faithfully remade for the 2017 Christmas special "Twice Upon a Time", which itself takes place within the final moments of this serial and expands on the circumstances behind One's regeneration.
- Absentee Actor: William Hartnell sits out the third episode due to illness. As a result, his lines were assigned to Ben and other characters.
- AcCENT upon the Wrong SylLABle: Theeee CYbermen have A biZARRE singSONG ACcent.
- Air-Vent Passageway. Used by Ben to get to the missile to sabotage it. According to the DVD Commentary, the first example in the history of Doctor Who.
- Aliens Steal Cable: The original concept for the Cybermen was that they had learned human speech through radio and TV broadcasts, which explains their strange, synthesized speech and the way they opened their mouths and dispensed it like a recording (weirdly presaging automated phone robots). It wasn't explained on screen, however, and subsequent appearances did not follow up on the idea.
- All Planets Are Earthlike: Being Earth's twin, Mondas has continents identical to ours.
- All There in the Manual: Later expanded universe sources, most notably Doctor Who: Cybermen and its audio adaptation The ArcHive Tapes, have In-Universe historians identify the Cybermen seen here as CyberMondasians (or CyberMondans), which leads to the Twelfth Doctor being able to recognise their origin on sight. That more advanced Cybermen appear to have invaded Earth earlier is explained as being a group of Mondasian Cybermen called the CyberFaction who took their upgrades Up to Eleven and left Mondas for space, while the remaining Mondasians only replaced what was needed.
- All There in the Script: The script to the story says that the power drain from Mondas is also the cause of The Doctor's weakness (and thus regeneration), but this is never actually stated in the story itself. A better explanation as to what's happening to One is given in "Twice Upon A Time".
- All Your Base Are Belong to Us: Twice at Snowcap, and at the space agency HQ and reportedly other major government and military sites in the final episode.
- Armor-Piercing Response: The lead Cyberman makes a point worth reflecting upon when Polly asks why he doesn't care that some humans are going to die without help."I do not understand you. There are people dying all over your world, yet you do not care about them."
- Apocalypse How: Somewhere between Classes 4 and X, according to the Cybermen, although the Cybermen offer to rescue what humans they can as long as they accept Cyberconversion once they get to Mondas. Mondas itself suffers a Class X at the conclusion of the story.
- Artistic Licence Astronomy: Exactly how the Z-Bomb would make Mondas explode like a supernova is never explained, nor is there any major gravitational effect on the Earth from a planet of equal size coming close enough to distort the orbits of spacecraft in low earth orbit, or how the disintegration of said planet left the Earth unaffected.
- Benched Hero: Happens to William Hartnell frequently during the third season, but it is especially apparent here. The Doctor is absent entirely from Episode 3 due to Hartnell's health that week, and even in the episodes where he appears his role is much reduced as he sits on the sidelines for much of the story, held prisoner either by the Cybermen or by General Cutler. The overall effect works in the story's favour, as it emphasises the Doctor's near-death exhaustion and fatalism. In addition to Hartnell's health, another factor was that he had already been removed from the series—this was actually the first story of Patrick Troughton's first production block note , and Hartnell was hired solely as a guest star. The scripts were written to not ask too much of him, given what was expected to be his mixed feelings about guest starring on his own show.
- Bittersweet Ending: The Cybermen were defeated and the Doctor once again saved the day, but at the end of the serial, he regenerates for the first time.
- Blue-and-Orange Morality: The Cybermen at least started out as having a morality which felt alien to humans, thanks to their emotionless logic. Here, they intend to destroy Earth. When Polly screams out that they are killing people, the Cyberman merely points out how illogical her outburst is, as people die all over the world constantly, and she does not display any distress over that. The Cybermen are not even actively malicious in the story. Their survival simply means that Earth has to be destroyed, so they set about to do that. They even make an offer to the nearby humans of being able to continue their existence as Cybermen, which seemed like a perfectly reasonable proposal to the Cybermen themselves.
- Body Horror: The point of the Cybermen's cloth faces and bare arms.
- Characterization Marches On: The Cyber-voices changed to an ordinary Machine Monotone because these original voices, with unusual pauses and inflections, made it hard for the actors to tell where their lines began or ended.
- Chest Blaster: Actually a gun kept on a chest bracket when not it use. And a bit lower than the chest, which sometimes leads to unintentional humour.
- Colourblind Casting: Williams was originally Welsh. Director Derek Martinus cast Bermudan actor Earl Cameron in the role, deciding that he could be black and Welsh. This is a stroke of genius, as black people have settled in the dockland areas of Cardiff since the 1850's.
- Creepy Monotone: Quasi-example the Cybermen talk in bizarre, alternating, almost singsong tones which nonetheless carry no emotion.
- Culture Clash: The Cybermen genuinely don't understand why the Humans don't wish to cooperate with their plans.
- Early Instalment Character Design Difference: The Cybermen in this story have many more visible humanoid parts than in any later story, making them look less streamlined but adding extra Body Horror.
- Earth-Shattering Kaboom: The major threat for both planets during the final two episodes.
- Electronic Speech Impediment: The Cybermen speak in a creepy, broken singsong voice.
- Exty Years from Now: The story is set twenty years in the future.
- Facial Horror: The Cybermen have cloth facemasks, black void eyes, and mouths that just sort of hang open while they talk, implying severe facial disfigurement going on underneath. It's part of why they are so disturbing.
- Fake Shemp: The "Doctor passes out at the start of an episode for no clearly explained reason" plot device gets one last outing at the start of Episode 3. William Hartnell collapsed with bronchitis and had to take a week off. As a result, Hartnell's regular body double Gordon Craig plays the Doctor for Episode 3, and the lines originally written for the Doctor were given to Ben, Polly and Barclay. Craig also doubles for Hartnell in the "exterior" scenes in Episode 1 as the TARDIS crew are arrested and taken into the base. Hartnell's absence for Episode 3 actually works in the story's favour, as it means the Doctor summoned enough energy for a "once more unto the breach" and you can tell he's dying as Episode 4 progresses.
- Five Rounds Rapid: Averted! The Cybermen are Immune to Bullets, but the Snowcap personnel quickly manage to get hold of some Cyberguns, which work perfectly well. Played straight, however, with Cutler's death.
- General Ripper: General Cutler is a perfect Ripper type. He wants to destroy the cybermen's planet with a planet-busting "Z-Bomb" to avenge his own son and is ready to disobey orders to do it (the Doctor realizes that doing nothing is the better course of action; and most of the story is his companions trying to stop the out-of-control general).
- Heroic Fatigue: The Doctor is out of commission for Episode Three. He rallies briefly in Episode Four, but is clearly exhausted by the end, staggering back to the TARDIS to set the ship in motion before he regenerates.
- Homeworld Evacuation: To try and clear up the Cybermen's Multiple-Choice Past, the novelization says that they originated on the planet Telos and had to evacuate to Mondas.
- Innocuously Important Episode: It has three main points about it that get very important later. It introduced the "Base under Siege" formula that would dominate the Second Doctor era and influence the show's slide from a Genre Roulette format into Monster of the Week; introduced the Cybermen, who would become one of the most iconic villains in the series; and ended with a shock twist of the Doctor suddenly turning into a totally different person, with a new actor in the role. All of these at the time were just decisions being made for this particular story and Real Life Writes the Plot, but due to the show's Kudzu Plot nature all became very significant (although some in terms of the show's feel rather than in plot points).
- Episode 3 is the last surviving episode from the Hartnell era, and Hartnell's not in it.
- And the episode that features him regenerating is the only one missing from this story (though it got reconstructed rather beautifully)
- The story that is supposed to focus on the demise of the First Doctor doesn't do much to explain why he's dying or give him much of a sendoff- he just admits he's growing weak and then drops down dying at the end. Instead, the story that actually focuses on him wrestling with the prospects of his life ending is explored over 50 years later, long after the death of the original actor and overlapping with the end of another Doctor's tenure who is a spiritual successor to how the First Doctor generally composed himself.
- Keystone Army: The Cybermen on Earth still draw their power from Mondas and stop moving when it's destroyed.
- Matrix Raining Code: The serial uses this in its Episode Title Cards, evoking listening stations.
- Multinational Team: At both Snowcap and the space agency headquarters.
- My God, What Have I Done?: Ben after he kills a Cyberman in self-defence.
- National Stereotypes: The Italian soldier has pin-up photos all over his bunk and perves over Polly, and the American General is a trigger-happy nut.
- No Ontological Inertia: Played with: the Cybermen draw power from Mondas, thus it makes sense that its destruction means their death. But rather than simply falling down dead when Mondas is destroyed, their organic parts out-and-out disintegrate. While it's said they were wholly dependent on power from Mondas, thus explaining why they died alongside it, not only do they drop dead almost instantly, but their organic parts, for almost no reason, disintegrate completely.
- Novelization: Was novelized as Doctor Who And The Tenth Planet by the televised story's script writer, Gerry Davis.
- The Nth Doctor: The first notable example.
- Nuke 'em: Cutler's plan in Episode 3, despite what would have been catastrophic results for Earth.
- Off Screen Moment Of Awesome: While Ben and Polly are trying to catch up with him at the end of the story, the First Doctor has bumped into the Twelfth Doctor.
- Out-of-Genre Experience: This is a First Doctor story that feels strongly like a Second Doctor story. It follows the "Base Under Siege" format associated with the Second Doctor era and features the Cybermen, a villain used heavily in Patrick Troughton's tenure.
- Panty Shot: In Episode Three, when Polly climbs up into the bunk, you get a flash of her knickers.
- People in Rubber Suits: The original Cybermen were guys in a balaclava and a (somewhat loose) catsuit, carrying plastic stuff suspenders-like. Plus a funnel on their heads. And a serious speech impediment. It actually worked; they were terrifying instead of being ridiculous.
- Pinball Protagonist: The First Doctor ironically is this in his last story, but the script plays with this trope in an interesting way. The Doctor has limited screentime in the story because William Hartnell's health was failing, and even spends a whole middle episode asleep (apparently for no reason- until five decades later a proper explanation comes into play that his body is gravely weak from old age. His plan for dealing with the evil planet draining the Earth's energy is incredibly passive: simply to wait for it to die, which he says it will do in a couple of hours. Unfortunately, his expansive apparent knowledge followed by his sudden absence ramps up the paranoia among the humans to fever pitch to the point where everyone turns against him and the General even accuses him of killing his son. Even after his prediction turns out to be right and the planet dies, it's a hollow victory, as the Doctor's unconsciousness is revealed to be a Chekhov's Gun foreshadowing his regeneration.
- Ripped from the Headlines: Kit Pedler was asked to write about something in the science world that concerned him. Early robotic prosthetic limbs and implantable artificial pacemakers were emerging at the time and so he chose to create a race of Cyborgs that had replaced most of their human bodies with machinery.
- Setting Update: The 1976 novelization changes the year from 1986 to 2000.
- "Shaggy Dog" Story: General Cutler really goes off the deep end when he thinks his son has been killed. He tries to kills the Doctor, whom he blames for the disaster, only to be killed by the Cybermen. It's later revealed that his son is alive and well.
- Sheathe Your Sword: Mondas blows up on its own, killing all the Cybermen, and Cutler's attempts to take more aggressive action would have led only to disaster.
- Shout-Out: Ben calls Polly "Nanook of the North" because of her fur coat.
- Special Edition Title: The story title, episode number and writer are given in a specially made sequence '60s computer graphic sequence, unique to this story.
- Stay in the Kitchen: The space programme has men from all over the world (as opposed to a purely American or British programme) but no women whatsoever.
- Tap on the Head: Ben gets knocked out after sabotaging the missile and has to watch the countdown when he wakes up not able to remember whether he did it or not.
- Tempting Fate: The news broadcast reporting that there is absolutely no danger from the mysterious new planet.
- Thicker Than Water: Cutler's more irresponsible actions are driven by the desire to save his son.
- Villainous Rescue: Cutler is on the point of murdering the Doctor and friends when the second Cyberman squad burst into the base and kill him. The Doctor is generous enough to thank them. Also a big part of their plans for the people of Earth, for a given value of "rescue."
- Weaksauce Weakness: These Cybermen are much more vulnerable than humans to radiation, despite their bionics.
- Well-Intentioned Extremist: How the Cybermen got the way they are.
- What Measure Is a Non-Human?: Averted: the Cybermen here uniquely have individual names, and Ben feels guilty after killing one of them in self-defence.
- Wham Episode: Introduction of the Cybermen, regeneration debuts, Hartnell leaves.
- Wham Shot: The final moments of Episode 4, in which the Doctor regenerates for the first time.
- Wins by Doing Absolutely Nothing: The Doctor ultimately does nothing and the Cybermen and their planet are destroyed by absorbing too much energy. The invasion would have played out exactly the same way if the Doctor and his companions had not been there.
- Writer on Board: Dr. Kit Pedler based the Cybermen on his fear of dehumanizing medicine — specifically mechanical things such as pacemakers.
- You Can't Fight Fate: The first time that this trope is applied to something that the Doctor evidently knows as being a historical fact, but which is still in the "future" of the viewers watching at home. Once he recognises where he is and what's going on, the Doctor subsequently treats all events in the story as being inevitable. Of course, since the power drain will destroy Mondas before it threatens Earth, and the plan to destroy Earth to stop the drain in time requires human help (apparently, they never invented radiation suits) the Doctor knows that the proper course is to sit back and wait for the Mondasians' time to run out. It's less "you can't fight fate" and more "don't screw with fate when it's on your side." Though near the end they must stall to prevent the Cybermen from killing anyone else, most of the drama isn't fighting the Cybermen, but stopping General Cutler from using his doomsday weapons that could leave Earth an irradiated wasteland - or from killing the Doctor and company in punishment afterward.