Fridge logic, if you’re not familiar with the term, describes the sort of plot hole that you only realise when the (e.g.) Doctor Who episode you’re watching is over, you go to the fridge for another beer (say), and then it hits you: why did they need to do that? The link goes to tvtropes.org, and if you’ve plenty of time I recommend you following it. I do warn you, though, that it will eat up your time and mind like wikipedia on crack. “Just one more link…”
Also, if you haven’t seen the latest Doctor Who Christmas episode, “The Next Doctor”, the next bit will be spoilerriffic. I’ll start off vague and get increasingly specific, so if you don’t want to know the score, look away now. I experimented with spoiler tags, but that just got annoying.
So the first bit is simple: if the cybermen have a gigantic dreadnought-class mecha robot almost ready to stomp over all of London, and all it takes to power it up fully is for a few dozen urchins to fuss over treadmills for at most a couple of hours, while the Doctor works out how to infiltrate the secret base, why couldn’t the cybermen just man the treadmills themselves?
Also, what the hell is the matter with Russell T Davies and applied phlebotinum? We have the cybermen dumped in the past resorting to primitive technology - fair enough. You can frob it to get it to tell you what it knows about a certain subject - also fair enough. If you break it in the right way - which everyone seems to be able to do - you can turn it into a Cybermen-killing ray-gun, by dint of overloading them with unwanted information. (Never mind why someone would design an information capsule to contain enough energy to be able to transmit information at lethal speeds at a distance, when it’s designed to be stuck into a chest cavity.) Hell, you can even turn it into an apotheosis gun, where you use multiple capsules at the same time, aimed at newly-formed Cyber-King, to remind her of her humanity and what she’d forgotten, as if such information would be in the cybermen’s data storage in the first place.
(Oh, and who exactly eats Christmas dinner at around 1am in the morning in Victorian London?)
Also, I couldn’t help wondering, why why does fake Doctor’s wife, in the flashback, look so similar to our antagonist? (Who was wonderful and the best thing about this episode.) And when the Doctor suggests that there’s something really momentous that prompted the fugue state, I was really hoping that it would be something more interesting than “your family has really suffered”.
Given all of this, here’s what I think would have made a better
episode two-parter. (Yeah, so you couldn’t do it as a cheap Christmas one-off.)
The Cybermen tumble out of the void, just a few of them, armed with only that Dalek technology and a few information capsules. They arrive in the cellar of a house that a couple from out of town, Jackson and Mercy Lake, have just moved into, along with their young child. The cybermen round up the humans and start the conversion process, as they desperately need greater numbers. While the conversion process is underway, one of the cybermen starts going through the information capsules, uploading the stored information into their shared consciousness.
Except it all goes horribly wrong: Mercy has a mind that’s too strong for the cybermen, especially in their reduced numbers, and the conversion process goes haywire. Jackson’s mind is already wiped, but there’s nothing left to put in it - except the knowledge of the Doctor that’s currently sloshing around the cybermen’s minds. In the confusion Jackson escapes, in a fugue state, not knowing who he is, forgetting his wife and son, remembering only the Doctor.
The plot develops as before, except that the skulduggery involves factories and their precious metal needed to construct the dreadnought, not that orphan nonsense. And the confrontation at the end can involve Jackson Lake, facing up to his wife and confronting her with her own suppressed memories, rather than the Doctor waving a shining glow stick of enlightenment at her.