Whilst Tenth Planet may have been the first time we encountered the Cybermen on screen, and the First Doctor the first of our heros to battle them, the Fifth Doctor (along with Nyssa) were there at their creation. They were there on Mondas at the birth of the Cybermen, and that tale is told thanks to Nicholas Briggs and all at Big Finish.
Unlike the origin story of the Daleks, who were mutants, the Mondasians were very much human. Spare Parts thus became a story of how humans could become so desperate that they would reject their humanity – even their emotions – to survive.
The Doctor and Nyssa arrive on a frozen planet, in a city where cybernetic implants are all that is keeping the population from death. The Doctor knows this planet all too well, and refuses to interfere in events that will become the birth of the Cybermen.
That planet, as we all know, was Mondas. The home planet of the colonised race of humans set to become one of our greatest foes. And the moral argument between the Fifth Doctor and Nyssa raged through this tale, making it one of, if not the, best Big Finish audios ever produced. In this authors opinion, any way.
Given what happened since, in The Tenth Planet and beyond (including further Big Finish Cyber adventures such as Human Resources and The Blue Tooth), maybe the Fifth Doctor could have changed the future of the Mondasians, maybe helped them find an alternative way to survive. Showed them that adapting themselves with spare parts was not the way, as Nyssa knew from the moment she realised where they were when they stepped out of the TARDIS.
Without giving too much away, despite their best efforts, the Doctor and Nyssa did ultimately, fail to change the future. But the fight they had trying was enthralling.
On a side note, Spare Parts was partially adapted into the television episodes “Rise of the Cybermen” and “The Age of Steel” although, in actuality, the two stories bear little resemblance to each other.