'Spare Parts' is one of those stories that's almost, almost, almost legendary. It's very good, don't get me wrong, and certainly quite well-remembered; Marc Platt is one of my favorite 'Doctor Who' authors, and this is one of his better works. But you can feel the story pulling back just a little at the end, because it has to risk being hated to be perfect. And it's just a little bit too afraid of not being loved to do what it needs to do to be legendary.
The story is, in structure, similar to 'The Aztecs'; the Doctor and his companions travel to a historical civilization that straddles the line between brutality and glory, and must choose whether to use their power to change that civilization to eliminate the brutality, or to keep history on its established course...only to discover that they don't have quite as much choice in the matter as they thought. The only difference is that here, the "historical" civilization exists in Doctor Who's history, not the textbooks; this is Mondas just before it became the home of the Cybermen, monsters who've been a part of Doctor Who almost since the beginning.
Like 'The Aztecs', 'Spare Parts' gives us a well-grounded, sympathetic portrayal of the society the Doctor and his companions enter; we meet the Hartley family, some of the most charming and likeable characters in the history of Doctor Who, and Doctorman Allen, who has to be the most likeable and sympathetic total bastard Doctor Who has ever given us. (She's not quite Bester, but she's damned close.) We see how a pre-Cybermen Mondas might work, and for the first time we're really given to understand the dilemma that led the Mondasians to become Cybermen. Doctorman Allen at one point challenges the Doctor to come up with a solution to their problems, if he's so willing to sit in judgment on the one they came up with themselves, and his silence is telling.
Every character is great, really. Dodd is the kind of character who would be the villain of any other story, but here he's almost decent; he might be a literal organ-stealing mercenary serial killer, but even he's horrified by what Mondas is becoming. Sisterman Constant is a zealot and a prude, but you see her enough to know that she genuinely cares about her charges and believes that she's doing what's right. Even Zheng, the chilling harbinger of Mondas-To-Come, is doing the best he can to save his homeworld and its people. It's just that when you look at it in the cold light of logic, well...the only way to save everyone is to convert them. Nobody has ever shown the Cybermen's side of things before, and it's surprising how easy it is to agree with their methods even as you shy away from the ultimate extension of those methods.
Which is, unfortunately, the problem with 'Spare Parts'. Unlike 'The Aztecs', this story tries to have its cake and eat it too. It doesn't take its story to the ultimate conclusion we know is coming, the tragic ending that we saw from the beginning as the Doctor realizes he can't avert Mondas' horrible destiny. It's understandable, really; taking the story to its logical extension would mean sitting through a chilling, brual ending with the deaths of pretty much every character we've grown to care about. (As opposed to the deaths of pretty much every character we've grown to care about except three...it's hard to tell why they try so hard, given how bleak everything's been up until Part Four.)
As a result, the ending feels hollow. We all know the fragile peace won't last. We all know that the Cybermen are right from their perspective, and their perspective is the only one that matters to them. The stinger ending feels like it should have been the real ending, or else the Doctor should have been allowed to make a difference. Trying to do both doesn't quite achieve either. That's not to say the story isn't very, very good; every actor is amazing, the worldbuilding is fantastic, the dialogue is note-perfect. It's only at the very end that you feel the difference between the tragedy the story wanted to be and the adventure mold Doctor Who usually fits into.