An origin story for the Cybermen of Mondas, written by Marc Platt. The story expands on the backstory of the Cybermen as detailed in "The Tenth Planet" and "Attack of the Cybermen".This story was an inspiration of sorts for the Doctor Who episodes "Rise of the Cybermen"/"The Age of Steel" (on which Marc Platt got a story credit): one of the story's most powerful scenes (a partially-converted Cyberman doesn't understand what's been done to her) is drawn from this audio. The character of Yvonne Hartman, appearing in "Army Of Ghosts"/"Doomsday", is additionally a Composite Character of this story's Yvonne Hartley and Doctorman Allan.Beyond that, it's considered an absolute classic Doctor Who story.
Earth's long-lost twin planet, Mondas, drifts through interstellar space, while the nearly extinct human population live in the last populated underground city. Resources are nearly gone, health is poor for just about everyone, and organ replacement — both organic and manufactured — is commonplace. Events are already spiraling towards a final end for Mondasian civilization, but the approaching Cherrybowl Nebula may rip Mondas apart unless they can activate the untested planetary propulsion system.The Fifth Doctor and Nyssa land on Mondas. By the time the Doctor realises where and when they are, Nyssa has already befriended a sweet young woman named Yvonne Hartley and her family. The Doctor tells her that they're nothing they can do to save this planet, but Nyssa is still traumatised by the events of "Earthshock", and she decides to stay with the family during their final days. She also brings Yvonne's pet Cybermat into the TARDIS, which promptly starts chewing on wires, causing a fire. The Doctor has to really take a moment to step outside and regain his composure.Yvonne is drafted into the surface crew and taken away from her dad and brother. Since the surface's atmosphere is impossible to stay alive in, she's given a prototype suit and (due to a power failure) only partially converted. Since organ failure is everywhere, people have started upgrading their entire bodies just to stay alive... and then lobotomising the sick to help cope with their constant pain and grief. This, combined with the prototype suits for surface exploration, resulted in the very first Cybermen, who promptly saw themselves as superior beings and started trying to convert the rest of the population. The Doctor knows he shouldn't meddle, since You Can't Fight Fate, but his Adric-related Survivor Guilt causes him to investigate anyway, leading him to an unscrupulous organ dealer.Both are soon captured by Doctorman Allan, the inventor of the Cyber-suits. And when she realises that the Doctor has a body far more advanced than that of a human, she uses him as the blueprint for all Cybermen, which can now be converted with a far greater efficiency and with better chances of survival thanks to Allen's understanding of his Time Lord brain structure. Allen realises that she's dooming her own planet, but she did what she had to do for her country, and now sees it as her duty to at least dull the emotional pain of it all with institutionalised lobotomies.The Doctor gets really very shouty and decides to Screw Destiny, mixing bottles of Allen's booze into the nutrient feed of the techno-organic Mondas Central Commitee (the first Cyber-Planner) and hoping for a Rubber-Band History result. It doesn't work for long, of course, and as soon as he and Nyssa eventually leave, Mondas society starts to collapse in on itself.