Here's an an example of a basic premise that's sound, but the execution a bit short. The Vogons, they of the planet of Gold, and the Cybermen have a plan to eliminate the other, with the poor Humans (and our heroes) caught in the middle. But there are so many plot holes it's difficult to enjoy. For example, if the Vogons are so rich with deadly (to Cybermen) gold, why haven't they developed a effective weapon using it? What was Kellman's motive? Why can't Cybermen tie up humans effectively?
Combine that with mediocre acting and questionable special effects (though the cybermats were well-executed), you get an episode that doesn't quite work.
Combine that with mediocre acting and questionable special effects (though the cybermats were well-executed), you get an episode that doesn't quite work.