I've moaned in the past about just how often Clara is promoted to leading an episode, but "Flatline" seems to really make it work. Set predominantly in Bristol, the majority of the running time is spent following Clara and her unlikely gang of misfits as they attempt to survive while they figure out the cause of a series of disappearances in the local area. It's another Doctor-lite episode - The Doctor is trapped inside the functionally useless and rapidly shrinking TARDIS for nearly all the episode.
The actual concept of "Flatline" is that people are being transformed into two-dimensional objects by forces unknown, demonstrated by the pre-opening credits segment of the episode. It's an interesting concept, certainly unique in Doctor Who as far as I know, but in the end it's only used to provide another 'monster of the week' like set of villains for the Doctor to ultimately defeat. Although slightly underwhelming in that sense, the design of these monsters is amongst the best that the series has ever offered, with some excellent CGI being used to provide a creepy, unique enemy that seems genuinely dangerous, a feeling that is only heightened by The Doctors inability to take action throughout most of this episode.
Clara comes to the front naturally this time, taking leadership over those in peril and ushering them to safety. We've seen this version of Clara before, but not in the same way - this is Clara taking on the role of The Doctor when he can't do it, and it seems to be an important step in their relationship. It seems to me that this should have been one of the first episodes of the season, showing us the first time Clara got to be the hero, the one who saves everyone - instead, the episode comes most of the way through a season of Clara already being the hero, lessening the impact somewhat.
At the end of last episode, we see Clara unable to 'give up' The Doctor, and that's expanded on here - it seems that her job and boyfriend (in other words, real life) have become her 'alternate life', and her travels with the Doctor her main. This is something that's been touched on before with Rory and Amy in the Smith era, but not to the same extent - Clara has responsibilities to both herself and others that are put aside at a whim, and she is lying to everyone she has to in order to keep travelling, almost like it's an addiction. The focus on Clara this episode also allows us to see the impact that The Doctor has had on her, with a throw away line about her claiming to be The Doctor causing more than a little discomfort for the time lord in question - at first, because she is making fun of him, but later because he can see how she has changed. She's celebrating a victory rather than mourning a loss of life, and The Doctor can see what he would appear to consider the worst parts of himself in Clara now.
Special mention needs to be made of the special effects work in this episode as well. If some parts of season 8 has looked shoddy in the visual effects department before now (and they have), I can only assume it's because the whole budget was blown on this episode - it looks fantastic. The conversion of objects from three-dimensional to two-dimensional is astoundingly well done, as are the scenes where The Doctor has to interact with Clara through the tiny TARDIS, with care being taken to ensure that the scale of them both still makes sense as the episode continues. It really is the best CGI that Doctor Who has ever had, and it helps elevate an already good episode further.
We are 9 episodes into a 12 episode season now, and the show seems to be sprinting for the finish, with each new episode proving that the shows lower points ("Robot of Sherwood", I'm looking at you) are worth sitting through for the high points. And "Flatline" is one of these high points - an smart, well paced episode that provides a good story with enough room for character moments to develop naturally. The script is tight, the enemies frightening and the cast are, as always, fantastic, and any doubts I had about this season earlier on have been well and truly squashed by a string of episodes that hit all the right spots. It's not perfect, but it's good enough, and I'll be damned if that isn't what Doctor Who does best.