The TARDIS arrives at the South Pole in 1986. There is an International Space Command Station there, under the ice. A couple of astronauts discover a new planet. The Doctor passes a note to the space station's chief scientist Dr. Barclay because he has an idea of what it is. The space orbiter is being pulled off course by the new planet and its energy being drained. The new planet has land masses that look similar to Earth. It is the Earth's long lost twin, Mondas. Station commander, General Cutler doesn't listen. The Doctor warns that they will be getting visitors soon. Some of the soldiers are sent out to look at the TARDIS; meanwhile a flying saucer lands nearby. Massive silver figures approach the box and they kill the soldiers. An expressionless, bandaged face with black, sharklike eyes reaches down for a fur and looks at the camera.
They are still trying to bring the orbiter down; as they do this a group of fur clad people enter the room unnoticed. They throw off their furs, revealing themselves to be the silvery giants. Cutler says they must keep working to bring the astronauts down. The figure says - in a mechanical, emotionless, sing-song voice - that it is not important; the orbiter can never reach Earth anyway. Leading to this classic dialogue:
Polly: But don't you care?
Cyberman Krail: Care? No. Why should I care?
Polly: Because they are people and are going to die.
Cyberman Krail: I do not understand you. There are people dying all over your world but you do not care about them.
They reveal themselves to be Cybermen - humanoids who have replaced their body parts and eradicated emotions. Cutler is knocked out, whilst Barclay is forced to call off the alarm sent by the General. The station is allowed to try and save the astronauts. Ben makes a move for a gun but it is broken in half by a cyberman and he is taken away and locked in a projection room. Horrified, the station crew watch the destruction of the orbiter. The planet Mondas is nealy exhausted of energy and has come back to drain all the energy from Earth; all humans will be taken to Mondas and turned into Cybermen. Ben blinds the guard Cyberman with a film and, capturing its weapon, kills it. Ben returns to the control centre and Cutler takes the Cyber gun and kills the Cybermen. Cutler reports in to headquarters; he learns that a spaceship was launched to assist the lost astronauts, and it is manned by his son. Scanners pick up a fleet of ships heading towards Earth.
Cutler gets contact with his son. The Doctor suddenly collapses; he appears almost worn out. Cutler wants to use the Z-Bomb against Mondas in order to destroy it. HQ says no because the possible effects on Earth are unknown, but then Cutler asks if he is allowed to use any action he feels is required to stop the Cybermen. He uses the yes to go ahead with the Z-Bomb regardless. Ben argues that they should wait because Mondas will absorb too much energy; thus Mondas is in as much danger as the Earth, which explains why the Cybermen have landed. Ben is locked in with the Doctor, who is sleeping. Polly remains in the control room and utters that classic, when asked what she can do: "I could make some coffee or something." (See below.) Ben looks around the room and sees that handy of escape ploys - a ventilator shaft. Preperations continue apace on the Z-Bomb. Polly tries to persuade Barclay to stop the Z-Bomb. A team of soldiers are placed in readyness, with cyber guns, because more Cybermen are on the way. Barclay explains what Ben must do to stop the Z-Bomb; the ventilator leads to the rocket silo. The wave of Cybermen are destroyed. Butler realises Barclay has gone and prevents Ben's sabotage; the countdown begins.
The rocket fails to launch. The Doctor walks into the control room, looking refreshed. Polly asks what's happened. "It comes from an outside influence. This old body of mine is wearing a bit thin." Mondas is brightening and darkening, and Cutler Jr. cannot control his spaceship. More cybermen are approaching the base; Cutler doesn't care, as he is too concerned about his son. Cybermen enter the control room and kill Cutler. The new Cybermen look exactly the same and sound exactly like the first group. The Cybermen order the bomb disarmed and Polly is taken to the Cybership as hostage. Cybermen have landed all over the Earth. They are going to use the Z-Bomb themselves to destroy Earth. Ben realises that the Cybermen are not entering the silo; they are afraid of radioactivity. They play dead - the Cyberman dies upon entering. Ben and co. remain in the room, playing for time. The Doctor is taken to the Cybership; and an ultimatum is given to hand over the bomb or else the Doctor and Polly will be killed. Ben, Barclay and Dyson grab hold of reactor rods; the cybermen start to use gas. The radiation kills the cybermen. Ben uses a communicator to entice more Cybermen. Mondas starts to break up and the Cybermen collapse, dead. Cutler Jr. comes back online. Ben gets Polly and the Doctor. The Doctor looks frail. Ben says it's all over, to which the Doctor replies: "What did you say my boy? It's all over, that's what you said. No, it isn't. It's far from being all over. I must get back to the TARDIS, immediately."
The Doctor sets the controls and then falls to the floor. As the TARDIS dematerialises the Doctor's face begins to shine with light and his face changes in to that of a younger man with a mop of dark hair. It is the end of an era.
When William Hartnell decided to leave the show, the production team came up with that most brilliant of ideas: the Doctor could regenerate, changing appearance and characteristics. It solved the problem of explaining a new actor in one swoop and ensured the show's longevity (evil Mary Whitehouses and Michael Grades aside).
Call me a softy, but I got tears in my eyes at the regeneration; in 2007, I watched the episodes in order - on YouTube - from An Unearthly Child to the Tenth Planet. By the time I had reached "Tenth..." (and seen many stories and reconstructions I hadn't seen before) , I had grown enormously fond of Hartnell's Doctor, so it was really saddening to see him regenerate.
The story isn't the big send off it could have been because Hartnell was ill at the time. Apart from the regeneration, which must have been a shock in 1966, the story stands out because it introduces the Cybermen. The scenes where Anneka Wills talks to Krail are brilliant. She (and everyone else for that matter) is dwarfed by the Cybermen actors, who were all 6 foot plus. The Cyber voices are effective too, revealing their inhumanity.
There are shortcomings to the story. After looking strong, Polly suddenly gets reduced to making coffee; this sort of thing was to appear numerous times and herald the Dolly Bird era, where female companions looked pretty, screamed alot and wore mini-skirts, instead of being strong and intelligent like Barbara and Vicki. Cutler and the Z-Bomb were a bit OTT.
But at the end of the day, the story ends the Hartnell era in a good way.