That said, this episode was a great way to focus on the Clara without losing the Doctor. The Doctor stuck in the TARDIS while the companion has to do the work is one of those tropes that seems incredibly obvious but doesn't get trotted that often. The only other example I can think of is 'The Horns of Nimon' and the less we talk about that the better – partly because that review isn't out yet.
I think 'Flatline' is a spectacular concept. The 2D aliens is a great idea, and the visuals to accompany them were fantastic. The shrinking TARDIS was fun, and the Doctor's frustration at not understanding how someone could do that was well played, and his realisation worked brilliantly. I felt there was more talking to himself with impersonations too, but I couldn't quite put my finger on it. The community workers being those that needed saving was an interesting twist, and it was brave that one of the survivors was clearly the 'evil' one, and for the Doctor to quietly admit that maybe he shouldn't have.
But the brunt of this episode is Clara as the Doctor. And she steps up. She takes charge when she needs, and her whole 'very much a teacher' approach was a delight. She problem solves, and the mini-Doctor helps make it more believable as he can make the big jumps when it would be infeasible for her to do so. Clara's bit with the hairband was great, feeling very much like a Tennant moment, which is a little odd, though Smith was kinda close too. Her plan to recharge the TARDIS and utilize the talent of the 'delinquent' was spot-on.
Even the Doctor, in his grumpy new era, admits that Clara “made a mighty fine Doctor.” And she revelled in it, and she openly prodded him into admitting it. Which is where the important stuff happened. First she rejects a call from Danny, who she's been lying to this entire episode, while she badgers the Doctor to admit she did good. But he can't, he can admit a second time that she was “exceptional” but “goodness had nothing to do with it.” Which is theme running through a lot of New Who. Post-Time War, The Doctor struggles with whether he is a good man or not, simply because a lot of the decisions he has to make are a lesser evil over a greater one. That Clara had people die on her watch, and was then more bothered about the praise than the deaths was a shocking one. Especially to have it followed up with a watching Missy declaring that she chose well with Clara.
I think the ending was very much setting us up for the finale. Hopefully next week address her lying to Danny a bit more, as it felt skimmed over here. But nothing really grabbed me about this episode. I really struggled to write this review. I have a rule to not read other people's thoughts. I had to this time. Just to try and pin something down. That's not to say 'Flatline' is bad. Bad is practically easier to write about than good. This was just “decent” and “alright”. It's really hard to write about something that just doesn't generate any feeling. Though if this is the worst we get from this season, than we've had a bloody good season.