And then there are the times when the budget really hurts.
The episode starts well enough; Kala makes a convincing villain, and the scene where she plays the grieving widow contains some good acting on the part of Fiona Walker. It'd be tempting to overplay the sobs and telegraph the twist to the audience, but she delivers her grief very convincingly. She also does a good job of not over-emphasizing her inadvertent slip regarding Susan's whereabouts in order to clue in the audience, although Nation includes a scene that probably wasn't necessary where Barbara and company discuss the slip at length in order to make everything clear to the very tiny children watching the show.
That's not terrible in and of itself, but it does foreshadow a problem Nation has. He can't just have people figure things out and do something about them, he has to put in a scene where they hash out the logic behind their realization at great length for the slow of brain. Which, again, is not such a bad thing when you're explaining how Kala accidentally mentioned a tiny piece of information that she had no way of knowing about if she wasn't implicated in the theft/murder/kidnap plot, but...
...well, there's no getting around it. It is a lot more of a problem when your characters are discussing that something seemed a little bit off about Arbitan. Something suspicious. Perhaps it's that he acted like he didn't know Altos when the two of them had supposedly met? Yes, maybe that's it. It's the kind of subtle mistake that any master villain might make, the kind of slip-up that could ruin even the perfect plan of covering your ridiculously elaborate headgear with a hood that you can barely even fit over it, not bothering to disguise your voice even a little tiny bit or take off your heavy black gloves, and sit two feet away from the people you're trying to fool at an angle that doesn't really hide your features concocting a story on the fly about how you're radioactive and can't get up or come any closer.
Yeeeeee-ahhhhhh. The problem isn't just that this scene is terrible. It isn't even just that it's unbelievably, ludicrously terrible and it's the climax of the story. It's that if anyone had put even a tiny bit of thought into the sequence, it would have worked just fine. A line or two explaining that Yartek (who was otherwise a fine specimen of megalomaniac villain) couldn't take the suit off for some reason. A line explaining that he knew Ian would only give the key to the real Arbitan, and some sort of line about, "We can't take it by force--the key is too delicate, he'd break it." And instead of Ian's labored and idiotic-sounding discussion of whether or not there was something fishy about Arbitan's sudden foot-long head extension, an explanation on Ian's part that he knew all along that "Arbitan" was Yartek and gave him the fake key on purpose. (Which as it stands is even more unforgivably stupid, as the scene involves Ian being fooled but giving him a fake key anyway, apparently just for the lulz.)
That's the lesson to take away from this episode, and this story. Doctor Who can survive being cheap. It can survive being weird. It can survive random genre-hopping at every opportunity. It can even survive being a Flash Gordon pastiche with a railroad plot. But it absolutely cannot survive being this rock stupid.