Obviously I'm bringing this all up here because in many ways Ghost Light is Ace's show. The Doctor brings her to the site of one of her more traumatic memories, a hundred years before she burned the place down, because he's intrigued by the feelings she felt while in the abandoned building. There's definitely something alien at work here, even if it's not immediately clear what's happening -- and Ace is justifiably angry at the Doctor for bringing her here without telling her ahead of time (in fact, he sets things up as a puzzle for Ace to solve, which probably didn't make her feel any better when she learned the truth). And while the Doctor might be running things, instigating events to see what happens, it's Ace that we tend to follow through this story -- note how we experience almost nothing while she's asleep in part two, but instead hear some of the things that the Doctor's been up to after the fact.
And look, I've gotten this far without discussing Ghost Light's (in)famous reputation regarding its story. This story is well known as one that requires repeated viewings to fully comprehend (David J Howe described it as "Doctor Who for the video generation" in The Handbook: The Seventh Doctor) -- although to be perfectly honest I've never found the basic plot that difficult to grasp. No, what repeated viewings do is give a greater insight into what's happening, as additional pieces fall into place to give a more complete picture of events. Then the whole thing is wrapped in a dense layer of allusions, which both give a sense of pleasure when you catch them (so even if everything else is escaping you there's that at least) and contribute to the feeling that there's a lot going on here.
Like I said, the basic plot isn't too hard to follow: someone/something is running an experiment on evolution on Earth. Josiah Samuel Smith is the experimental subject, who seems to be continually evolving into a version of the dominant lifeform on the planet (let's just set aside the human-centric idea that we're the most advanced species on the planet). Control is (presumably) the control subject, but it's fed up with being locked away for such a long time, and it's ready to be released. Meanwhile the story has a lot to say about arguments between evolution and creationism (for lack of a better word). Or as Reverend Matthews puts it, "Mr Smith disputes man's rightful dominion over the forces of nature. ... Instead he maintains that mankind itself should adapt to serve nature or become extinct." The whole story wants to be an examination of evolution in some form -- although at this stage there are still anomalous details (such as why all the insects start moving around -- this will explained (sort of) in part three).
Really, though, these two episodes are filled with enough to charm and entertain the viewers that even if you can't follow the story, there's still plenty to enjoy (anything with Nimrod or Redvers Fenn-Cooper, the way the TARDIS has materialized with the door against a wall -- a gag they'd somehow never done before, the intensely creepy way Gwendoline is ready and willing to send the Reverend Matthews to Java (aka do something horrible to him)...). That said, it feels like there's still a lot to get through, and only one episode left to do it -- how will they manage?