It seems to have been a very long time since the last two-parter Doctor Who episode, with Moffat himself declaring way back that these elongated episodes were the main problem with Doctor Who, despite them frequently being the best episodes that Doctor Who has ever offered. And now, Moffat has gone and ended season 8 on a two-parter that he wrote himself.
"Dark Water" is the penultimate episode of Season 8 of Doctor Who, and naturally expands on what's been teased throughout the last 10 episodes. The spoilers start almost as soon as the episode does and don't stop for the whole run time, so details are going to be light - but this episode deals with the mystery of Missy and The Promised Land/Nethershpere, the recurring plot thread that has been dangling all season.
A common complaint of season 8 has been that Clara and The Doctor really don't seem to get on that well, and that it's always seemed that The Doctor doesn't really like Clara very much, but "Dark Water" goes to some lengths to fix that. It's a little bit jarring to have The Doctor loudly proclaim how much he cares about her after a season of indifference at best, but the "fix" in the relationship works in the episodes favour, allowing "Dark Water" to focus on the plot at hand by removing the ambiguity that has surrounded their relationship for so long now. We get a great scene that really demonstrates how the Clara/Doctor relationship works that also allows us to glimpse through Capaldi's Doctors hard outer shell and see the more humane side of him, an important moment that has really come far too late in the season.
There is a lot about the actual plot of "Dark Water" that I really liked, but so much of it is about the plot or some of the reveals that I'm simply going to stay quiet about it. However, what I will say is that this may be the strongest motivation for an episode to date - both Clara and The Doctor are heavily invested in the events of this episode for one reason or another, and this helps give "Dark Water" a sense of urgency and seriousness that the show often lacks. We also finally get to learn more about Danny Pink and the part of himself that he doesn't like to talk about, and although that reveal is particularly predictable, it's handled well, and looks like it will actually matter to the story going on.
The plot mostly develops through characters simply talking to one another, but the dialogue is well written and feels natural, meaning that we never end up listening to someone simply explaining the plot. The Doctor, Clara and Danny all end up fulfilling the role of audience substitute for large parts of this episode as they know as little about what is going on as us, and it gives "Dark Water" a great mystery feeling to it - The Doctor and Clara are investigating an unknown entity here, and the episode builds up tension right up until the end.
The actual idea behind the The Promised Land/Nethersphere that has been teased all season is an incredibly uncomfortable concept to deal with, the kind of thing that sticks with you long after the episode has finished. It's a great example of Moffats ability to make the ordinary become disturbing, taking a basic fear of the unknown and expanding on it to great effect. It almost seems too dark for Doctor Who, and I wouldn't be overly surprised if the finale of the episode somehow reverses this idea to undo what is implied here, because it really is one of the darkest, terrifying concepts that Doctor Who has ever used.
The only real complaint I would have is that the Missy reveal is uninspired and really quite predictable, with people having correctly guessed who she is from the very first episode of this season. Other than that, "Dark Water" is one of the best episodes that season 8 has produced thanks to a slower pace that allows the episode to build momentum while exploring the concepts that it introduces and redefines throughout. It's great to see another episode that really demonstrate just how good Doctor Who can be when it's at it's best, and if next weeks finale can build upon the groundwork laid here, it should be an absolute blast.