This is the first part of a 2-part story and follows on from plotlines introduced in the 50th Anniversary special The Day of the Doctor in which Zygons were allowed to live on Earth disguised as humans.
Here we find that a faction of the Zygons is rebelling, looking to attack, take control and return to Zygon form. We are shown that one of the Osgoods has survived and will not reveal whether she is the human or Zygon version. She has taken matters into her own hands to try to stop things reaching massive loss of life.
The Zygons are a great monster and are depicted well in this story and the threat presented is suitably menacing. UNITs involvement is well used and it is always great to have Jemma Redgrave appearing as Kate Lethbridge-Stewart and Ingrid Oliver as Osgood who are both engaging in their roles.
Clara is used well in the story although I find it a little annoying when she takes command of situations and gives orders to supposedly experienced military personnel rather than just giving advice or suggestions. It is also a little annoying having the slightly silly ideas of the President of Earth and Air Force One used again.
Peter Capaldi is on top form in this whole story and even sells the slightly silly bits of humour in a school playground or aboard his Presidential plane. His quality is unquestionable.
Writer Peter Harness wrote the daft and very illogical Kill the Moon but has a complete turnaround in this story where the writing is extremely entertaining and high quality. The script is strong and the plot is interesting. There are clear political messages (a pretty common Doctor Who theme despite what some fans like to make out) and these messages are good moral aspects to explore. It is well executed and enjoyable.
There is humour, intrigue, action and danger with Capaldi's brilliant Doctor at the centre of it all so it is very good indeed with no significant negatives for me but I wasn't overly keen on the setup or execution of the cliffhanger.
My Rating: 9/10.
Series 9 Episode Ranking: 4th out of 14.