(While this review does not contain major spoilers or specifics, some minor things may be given away. It also draws your attention to hints in the episode. Read at your own discretion.)
This episode finally introduces Danny Pink to the Doctor, the TARDIS and Clara's real life as we re-enter the comedic side of the Doctor this series, which isn't a new approach for writer Gareth Roberts whose previous episodes include The Lodger, Closing Time and Attack of the Graske, among several The Sarah Jane Adventures episodes.
This episode is also, again, co-written by Steven Moffat which means it brings the usual surprises and controversies, such as the increasingly frequent religious mentions that we're seeing with his episodes.
For such a long-time fan as Moffat is, the amount of references in this episode is surprisingly low, just a mere one or two to the fact that he's been there before, which is disappointing to those long-term fans. We don't even get to see the school sign from the 50th, which had more on it than the entire episode. While normally this wouldn't be important, the fact that this episode was set at Coal Hill school is a huge draw of the episode to a lot of viewers who would be expecting a lot of things. These fans will be disappointed.
This episode puts aside the baddie for the majority of the episode and focuses on the trio of relationships we see. Like most episodes where the big baddie is put aside, it is surprisingly enjoyable despite absolutely nothing actually happening for a long while but once we get past this stage and return to the action, the pace picks up and it gets more exciting and fun to watch and the now "well....that was adequate" feeling runs away.
In the opening, we see the relationship between Clara and Danny suddenly jump from the dating every now and then to them as a couple, so much so that the kid's parents even know about it. This massive step is then offset by spending the entire episode introducing one man to another, which is quite offputting. Coleman and Anderson get plenty of material to play with here and pull it off without a hitch, with a massive range of emotions from both of them, with the Matt Smith lookalike plonked in there for rather obvious reasons, which is pulled off really well.
Due to the focus on Clara/Danny, Capaldi's role is very much shoved to one side and as such he manages to go too far and, for the first time, begins to get a little un-likeable as The Doctor. Being a bit of an arse to a child is one thing, but his unyielding view of Danny as a soldier is far too prejudiced for a character where one of the main draws of the entire show is how open minded he is. This is the same man who remembered enough about his past to mention being in a fight with River, but not about the Brigadier, whom he fought alongside for in many adventures, or Wilfred, a soldier who he gave his life for. As fun as his insults and confusion may be, they got old really fast.
While Murray Gold's music has been somewhat spotty this series, this episode is definitely one of the highlights of the series. It has, almost literally, everything in there, including what sounds like a theme for Danny Pink (although it sounds surprisingly like Amy's theme). Although we still lack a good, clear, solid 12th Doctor theme, but this could be the point?
While this episode is far from winning any awards and is basically a giant filler episode for introducing Danny and tying up some loose ends from the previous four episodes, with a lot of humour added to the first half, and a bit to the second.
It's clear that this episode lacks a proper plot, but rather just showing the introduction on TV because they can't just simply skip it and therefore must make a monster out of it to make it at least reasonably interesting. This is especially clear when the BBC decide to release a promotional picture showing how The Doctor defeats the episode's villain (not included in this review).
There's no deep-thinking. No real scaryness. Just a fun filler. It's a perfectly watch-able episode that you could watch multiple times, but wouldn't particularly want to. Think Time of the Doctor. Overall, I would rate this episode 5.5/10.