It has everything that a doctor who episode should have. The best in years. One of the best of all time.
It reminds me the old classic Doctor who. Just a nice well written story and good actors to support it. No plot holes, everything is explained, full of action and suspense.
The first episode was a bit lacking, but this episode came to fix that. If you enjoyed the 'Midnight' episode from the fourth season, then you'll love this one too.
This is the first time in years that I can't wait for the next episode and the season to begin.
I hope it continues just as well in the upcoming episodes!