In 2070, on the weather control base on the Moon, the Gravitron which keeps Earth's weather in alignment is malfunctioning, the crew are falling sick of a strange disease that leaves odd dark streams through their faces and hands, and the sick crew members are disappearing. Also, someone is monitoring their transmissions to Earth.
While the base's doctor has fallen prey to the disease, one of a small group of travelers from Earth (so they say, but they're human as far as we can see) is a doctor himself. He offers to help, but the irritable and suspicious head of the base wants him and his friends to go. The little man called the Doctor refuses to go:
HOBSON: That's as maybe. I don't know who you are, what you are or where you come from. But you can get off the moon now.Polly, who has experience, believes that she has seen a gleaming silver man--a Cyberman--take one of the patients away. But that can't be:
BEN: Yeah, well that suits me fine. The sooner the better.
DOCTOR: No, Ben. We can't go yet.
BEN: Well, why not? They don't want us here.
DOCTOR: Because there is something evil here and we must stay.
HOBSON: Evil? Don't be daft.
DOCTOR: Evil is what I meant. There are some corners of the universe which have bred the most terrible things. Things which act against everything that we believe in. They must be fought.
HOBSON: We'll see about that in a minute. This thing you saw, what was it like?Of course it's a Cyberman, and of course Polly is right, even though she can't accept it.
POLLY: It was enormous and silver, and it had holes in it's head for eyes, like a robot!
HOBSON: A robot?
BEN: But the Cybermen were all killed when Mondas blew up, weren't they?
HOBSON: Stop this Cyberman nonsense. There were Cybermen, every child knows that, but they were all destroyed ages ago.
DOCTOR: So we all thought.
Sometimes we are so wedded to our notion of what is and is not possible that we cannot see the obvious, cannot feel the ground shift under our feet. Or rather, we can see it, we can feel it--we just don't let ourselves.
And that is what empowers the evil that threatens to swamp us: we refuse to perceive it, to acknowledge it. As Charles Baudelaire wrote, "do not ever forget, when you hear the progress of lights praised, that the loveliest trick of the Devil is to persuade you that he does not exist!”
"They always get started. They happen everywhere there's people. Mondas, Telos, Earth, Planet 14, Marinus. Like sewage and smartphones and Donald Trump, some things are just inevitable."--The Doctor.
The Doctor does not yet have the sense of inevitability toward the Cybermen in 1967 that he had 50 years later, in 2017. They are a surprise, though not a shock. When the Doctor realizes the one place that hadn't been checked was the sickbay n which they stand, he tries to get them out before--nah. Too late, the Cyberman throws away the sheet, and reveals himself, pointing a weapon and taking the Doctor, Ben, Polly, Hobson et al hostage.
Face-to-face with the adversaries that ended his first life, the Doctor is nonetheless recognized:
CYBERMAN 1: You are known to us.Subtle, a little ominous, but not much more.
DOCTOR: And you to me.
As the base is held captive, Jamie, who has been unconscious for most of the story rallies. Ben and Polly try to brainstorm ideas, and Jamie refers to sprinkling witches with holy water, a less than helpful idea, but one that sparks an inspiration in Polly--the Cybermen are made of metal, and plastic--which nail varnish can melt. So Poly and Benthey concoct a super-solvent, fill several squirt bottles with it, and sally forth to battle. (May I just say it's nice to see Polly mostly back on form after her rather unfortunate dumbing down in the last story.) And it works. The Cybermen inside the base are killed, and those outside are laying siege to the base--yeah. We're in the iconic Troughton "base under siege" plot line. As it's his first go at one of these, the story nips rather nimbly along. But the Cybermen are different then those the Doctor first met. The singsong voices, bandaged faces, are replaced by more typical robot like features, including an electronic voice that is damnably difficult to follow. They have less body horror, less dark magic than they were. They are also weaker in a very real sense--rather than conquerors certain of their position, offering conversion wholesale (which they view as an improvement), they are planning to destroy earth for no other reason than "to eliminate all dangers." They have learned fear, fear of Earth, that is. (Yes, they claim to be without emotion, but the Cyberman's sarcasm tends to undercut these claims.) Just like the Doctor, they have been altered through their first encounter, and are less formidable for the change.
It ends with the Cybermen (hopefully_) destroyed, and off to the next adventure. The stakes are a little lower than in their previous meeting, but the real takeaway of this story is that standing up and fighting against a foe becomes easier each time you do it. It only needs believing your own eyes, and remembering who you--and they--really are.