The other thing to note is that, although all of Pertwee exists on video, it's not all in the original format -- those tapes having been wiped and reused just like the black-and-white ones. So we get a hodgepodge of quality during the Pertwee years -- this particular story being a mix of a low-quality NTSC copy and a high-quality black-and-white film copy. It's a very good restoration, but you can still tell sometimes.
We're also introduced to that staple of the next few years: the isolated scientific research establishment. UNIT has been called in to investigate inexplicable power losses at Wenley Moor, which is running experiments on the nature of the atom. We're introduced to the head of the project, Dr. Lawrence, who seems pretty stressed out about the situation. We also meet the second-in-command, Dr. Quinn, who seems much happier -- but then it's soon clear, from his conversations with the perpetually unhappy-looking Miss Dawson, that he knows something about what's been happening at Wenley Moor. Major Baker, the chief of security, thinks it's internal sabotage, but UNIT doesn't seem so sure. The Doctor and Liz find enough to be concerned about things, but not enough to know what the nature of the problem is. But when they learn about a potholer named Spencer (who was attacked by a dinosaur in the opening shots of the episode), the Doctor decides to investigate the caves for himself, and is also attacked by a dinosaur for his pains.
Of course, as we see in episode 2, the dinosaur is called off before it can do any serious damage, and the Doctor returns to Wenley Moor to inform the Brigadier, who decides to take a party into the caves to investigate. Major Baker fires at a shadowy figure before being attacked by the dinosaur. Whatever Baker shot at, it's wounded. We get some nice first-person shots, with a reptilian claw visible and, intriguingly, a red filter over the top center third -- one can only guess at what the people with black-and-white televisions thought of that.
And we learn that Dr. Quinn is in fact working with these strange figures, who have a base in the caves and instruct Quinn to recover their wounded comrade. That comrade has, meanwhile, taken refuge in a barn, only to be discovered by the farmer. It kills the farmer and terrifies the wife, who eventually manages to tell the Doctor and the Brigadier that it's still in the barn -- the barn which Liz Shaw is currently investigating on her own...
So far it's been a slow burn, but these two episodes are paced very well, meaning that they remain highly watchable. There are quite a few questions being raised (What's causing the power losses? Who are the people in the caves? Why do they have a dinosaur? Why do they strike terror into humans? What's Quinn's relationship with them?), which are compelling enough to maintain interest for the next episodes. So far so good then.