This story was missing for nearly 25 years, until complete prints were uncovered in Hong Kong in 1992. It is not only the first complete Cyberman story, it is the first complete Patrick Troughton story. The seven previous Second Doctor stories have all been lost in whole or in part or completed with animation. It is also the first with Victoria (Deborah Watling) as a companion.
The episode begins with Victoria being shown around the TARDIS. She finds it all quite incomprehensible. The Doctor admits to being 450 years old. The TARDIS lands on the planet Telos, where an archaeological expedition has also landed, intending to wake the Cybermen believed to be buried there for centuries in underground tombs. The expedition is funded by Eric Kleig (George Pastell) and Kaftan (Shirley Cooklin) who plan to share power with the grateful Cybermen.
The expedition is a motley collection of adventurers that make you think of a bad Indiana Jones movie. There is an arrogant and sociopathic money man, a bearded and high-minded scientist, a scheming and ambitious female, a square-jawed and bravely earnest American space-pilot, a balding guy who is frightened by everything, and a powerful and noble black guy with a deep voice. The first guy who touches the door of the tomb dies instantly, but the Doctor, who naturally is mistrusted by the expedition, opens the doors for them.
Inside, they discover a sarcophagus chamber facing a deadly protective device. Victoria gets locked inside and must be freed by the Doctor. Experimenting with a control panel causes a Cyberman to emerge and a gun to fire, killing another member of the team. The Doctor learns that the room is a firing range and the appearing Cyberman a target-dummy. The rocket outside is suspiciously damaged, so the expedition cannot leave, though the scientist, feeling guilty because of the deaths so far, desperately wants to. The financier of the expedition and his female companion will not hear of it.
Deeper underground is a vast chamber with an army of frozen Cybermen in cells like so many insect larvae. Kleig revives the Cybermen and betrays everyone else. He believes that the Brotherhood of Logicians, of which he is a member, will be rewarded for freeing the Cybermen and allowed to rule the Earth. The Cyber Controller is awakened and takes most everyone prisoner. Others are attacked by Cybermats, who are crawling shelled creatures resembling a cross between a Cyberman and a lobster. The bad guys get what’s coming to them and the black guy dies nobly, sacrificing himself for the white people.
Eventually the Cybermen, after killing a few more expedition members, and scaring the bejesus out of everyone else, begin to run out of energy and return to their frozen tombs, and the Doctor causes an explosion that buries them, hopefully for another 500 years. The pitiful remainder of the expedition leave in the repaired rocket, and the time-travellers leave in the TARDIS. The Doctor fears they have not heard the last of the Cybermen, and indeed one escaped Cybermat is watching them leave.
The episode was criticised by some parents as too frightening for children, but it quickly became a classic, which is not surprising since it was one of only seven Patrick Troughton stories to survive the decade. But, to be fair, there was a lot to like in the scary scenes and interesting, if cliché secondary characterizations, if not in the plot, which seems rather thin. The thawing Cybermen clawing their way out of a wall of tombs to the sound of percussive and staccato music was impressive. It was filmed in one take.