Yeah, this is probably my favorite serial; it's either this or Tomb of the Cybermen. As you say, this is the kind of thing that I wish the original series did more. The mystery in particular is hinted at in most of the run, though only occasionally surfaces.

The acting is about as strong as it gets, here (especially the guest cast). The characters are about as evolved and interesting as they come, in the original series. The atmosphere is sort of the ideal that I think of when Doctor Who comes to mind. And then the pace and plotting -- I actually like it in three episodes. It's one of the only serials that I can pick out that goes off in its own direction and has the confidence to assume that I'll be able to follow it on my own. I like that; not having my hand held. And it's not like it's confusing on a basic level. If I miss out a detail here or there, it doesn't matter too much. Heck, it probably even helps the tone the story's trying to set.

You probably know that this story was originally to be Lungbarrow. Cartmel thought that was just a little too weird to pounce on the audience, though, so the story's focus was shifted to Ace instead of the Doctor.