This story is important because it was William Hartnell’s last, it first featured the Time-Lord regeneration process, and it introduced the Cybermen, a continuing adversary for The Doctors. Because of this, the missing fourth episode was restored with animation.
The TARDIS lands at the south pole of Earth in the year 1986. At Snowcap Base, under the command of General Cutler (Robert Beatty), everyone is watching the spaceship Zeus IV come in from probing Earth’s atmosphere. The ship is affected by a gravitational force and the observers see an unknown planet approaching Earth. The Doctor knows this is Mondas, the Earth’s long-lost twin.
A mysterious spaceship lands nearby and three creatures emerge, killing the guards and invading the base. These are Cybermen, a humanoid race who have replaced their bodies with mechanical parts and abandoned the weakness of human emotion. They are not the slightest bit bothered by the fact that their planet’s gravity has destroyed the Zeus IV. In fact, Mondas is absorbing energy from Earth and will destroy that too. The humans at the base will be taken to Mondas and made into Cybermen. Resistance is, in fact, futile.
The base personnel, with the help of the TARDIS crew, overpower the Cybermen and kill them with their own weapons. The General wants to hit Mondas with a Z-Bomb, at the risk of great loss of life on Earth. The Doctor collapses from exhaustion--Hartnell was genuinely suffering from bronchitis and in some scenes had been replaced by a stand-in wearing a white wig--and the General imprisons Ben with The Doctor for arguing against the bombing. But Ben escapes and sabotages the Z-Bomb rocket. General Cutler is about to execute Ben, The Doctor, and all who oppose him, but more Cybermen show up and kill the General. The Doctor, realizing that Mondas is doomed, offers the Cybermen a home on Earth. The Cybermen take Polly to their ship as a hostage.
What they are really planning is to destroy Earth with the captured Z-Bombs to save Mondas, and they order the humans into the bomb chamber to prepare the bombs. Ben thinks they did this because the Cybermen are more susceptible to radiation than humans, so the humans use radioactive rods to defeat the Cybermen.
The Doctor stumbles off through the snow alone. Ben and Polly follow him and find him collapsing on the console room floor. As they watch, he transforms into a younger man—200 years younger, in fact. Patrick Troughton momentarily makes an appearance. Nobody was sure how to change one face into another, and at the time it was a special effects triumph, though in the animated reconstruction of Part Four, it was ironically somewhat more realistic.
The Cybermen, despite their stocking-clad faces, were rather scary, coming out of a blizzard and speaking in their mechanical sing-song voices. Over time, their look became more streamlined and robotic. In a way, there are the opposite of the Daleks, who are stark raving mad in their steel suits. To my mind, their total lack of emotion makes them more frightening.