The second episode of Series 8 takes us someplace new. The Doctor and his traveling companions have been shrunk before (Let's Kill Hitler), but this time it is to journey into a Dalek to find out why it has turned good in order to do the same thing to other Daleks.
The episode pays homage to Fantastic Journey, with the Doctor even saying it would be a great idea for a movie, while at the same time being quite different and thought provoking.
We get to learn much more than ever before about how a Dalek works. We also get a nice comment from the Doctor (which applies to the character and the series) that it was his visit to Skaro that changed him.
The episodes also gives us some nice moments with Clara, both in her own life and in her travels with the Doctor. We also get an introduction to a fellow Coal Hill School teacher, Danny Pink.
Without giving too many more spoilers, I'll just say that this was a dark episode. The end was not what I expected, but when it came, it was exactly right. A wonderful episode that was over all too soon.