For some reason the Doctor agrees to help them enter the city. I wonder why, because the Doctor is also very extremely alarmed that they are going to enter this place. I'm sure the Doctor is concerned 'lest they wake up the Cybermen. It's best to let sleeping Robots lie than to wake them up and face another war. In any case inside the city they find a bunch of logic riddles controlling every door and other control panels. There are dangers galore from caskets for building Cybermen, to weapons practice ranges, etc, and plastered everywhere are Cybermen logos as if Cybermen Industries were in charge.
Turns out that Kaftan and Klieg have an ulterior motive. They wish to use the Cybermen as soldiers to lead a revolt upon humanity, so that the Logicians can take their rightful place as leaders of humanity. The fools. As logicians they've endeavored to act without their emotions, just as the Cybermen also have eradicated their own emotions, so the two must go well together.
Like a story of adventurers exploring an Egyptian tomb, one-by-one the archeologist crew are killed. There's even the panicky one who keeps exclaiming they'll all be killed, until he is mercifully killed.
One weapon used by the Cybermen are the Cybermats. These look like slugs, but are made of metal, can fly, can attack people, and generally they make the party very very afraid.
There is an underground catacomb where the Cybermen are frozen in sleep. They go underground and much to the alarm of the Doctor, Klieg begins the process of awakening them. Well what did you think dear Doctor would happen by helping these people get into the city.
Seems the Cybermen had run low on power and had put themselves into storage. They left the riddles upstairs thinking that for anybody to make their way through, they'd have to be awfully smart, and would make good Cybermen recruits. Well, that was their plan anyway. They must not have expected the one to make it through would be the Doctor.
In the first battle they're able to weaken the Cyberdudes enough that they have to retreat back into their tombs to conserve energy. The Cyber Controller is still on the loose however. For some reason the Doctor tries to trap it into the sarcophagus looking thing, but instead that revitalizes the Controller and now he's really dangerous. The Controller kills one of the conspirators, which then enrages this big hulking guy who was the bodyguard of that conspirator. He single-handedly kills the Controller somehow (er.. if Cybermen are so invincible then how could one human do in a Cyberman?).
In the second battle the Doctor had gone back underground because he had something to take care of, namely to make sure the Cybermen would not wake up. But the other conspirator has other ideas. But they manage to kill Klieg.
The only survivor is the lead archeologist, so in a way that one panicky archeologist was right. They all died (almost).
The story is notable for a very rare reference to the Doctor's family. In a discussion with Victoria about her dead father, the Doctor comments that his own family "sleeps in my memory", implying a dramatic separation of his own from his entire family.
I think the theme here is the quest for and uncovering of forbidden knowledge. Some forbidden knowledge is perhaps best left where it is. Revealing forbidden knowledge, though, does let you resolve it, because it can remain a danger tucked in the recesses of the universe until it is brought out and transformed.