There has been a peace treaty between humans and Zygons, with Zygons allowed to take a human form and live on Earth.
We learn that since the events of ‘The Day of the Doctor’, there were two Osgoods. One is the original, the other is a Zygon clone. However, both considered themselves to be the same. They saw themselves as like sisters or both as a hybrid of human and Zygon, and the pair came to symbolise the peace treaty between humans and Zygons. The Doctor left them both with the Osgood Box, which was never to be opened unless the peace treaty broke down.
However, one of the Osgoods was killed by Missy in ‘Death In Heaven’, and some time later the other was taken hostage by a rogue group of Zygons. Most Zygons want to live peacefully, having settled on Earth in their human disgusies. But some Zygons think that’s not enough, and they should all live openly in their true forms, and have formed a terrorist group.
They are even killing their own, including two high ranking Zygon commanders, who are killed in a video sent UNIT’s Tower of London headquarters. The video also shows Osgood reading a message which says that the Zygon terrorists want to launch an attack in Turmezistan. The Doctor goes there. There are already UNIT soldiers, and the Zygons have completely taken over a village. They also take the form of the soldiers loved ones to prevent them from killing them, and later to get the soldiers into a church hideout unarmed so they can kill them. The UNIT commander gives the Doctor a brief time to find and rescue Osgood before the village is bombed. The Doctor finds Osgood, but the place is bombed earlier than planned. They get on the plane back home, along with a Zygon who was about to attack them.
Meanwhile, Kate Stewart goes to the site where the video was filmed, from Truth and Consequences in New Mexico. She can only find a sheriff here, who tells her the Zygons stayed for a while, but when humans discovered their alien identity they turned on them, so the Zygons retaliated. It’s later revealed the sheriff is in fact a Zygon herself and we see her about to kill Kate.
Clara goes back to her block of flats and spots a little boy who is looking for his parents. Clara finds them, but they are acting suspiciously. Later, along with Jac from UNIT, Clara goes down the lift which leads them to an underground Zygon base. They get some UNIT soldiers to come with them, and discover lots of pods, which appear to have clones of people growing inside them. We see that one of the pods appears to contain a clone of Clara. Clara says they should kill the “clones”, but Jac realises that this isn’t how it works with Zygons. They don’t grow clones from pods, they don’t have to go to that sort of trouble. They simply morph into a copy of an original. So these people in the pods are the original humans. The Clara we see now is in fact a Zygon, and the real one is in the pod! She was taken by the Zygon clones of the boy’s parents. Jac and the UNIT soldiers are then killed by Zygons.
The Clara Zygon, who calls herself Bonnie, then finds the location of the plane the Doctor is on, and fires a rocket launcer at it!
The return of fan favourite Osgood was something which wasn’t exactly kept a secret. It was hinted at in trailers and pre-publicity for the ninth series, not just this episode. The question was more how they were going to explain her return, after all she was killed off in ‘Death In Heaven’. As it happened, the clues were already there. We knew there was a Zygon clone of Osgood in ‘The Day of the Doctor’. So is this Osgood the original or the clone? It almost doesn’t matter, as apparently both Osgoods began to see it as unimportant. But as an explanation for bringing the character back, it made sense from what had gone on before rather, and a lot better than some of the nonsense TV shows, particularly soap operas, have pulled out to explain why a character is “back from the dead”. Though talking of soap operas, I quite liked how Bonnie was like in soaps when they had an evil twin played by same actor. Clara turning out to be an evil clone was a great and unexpected twist.
Doctor Who once again has an episode with a very different tone. ‘The Zygon Invasion’ feels more like a gritty drama series than other episodes, with the focus on government, war and military action. It also has deliberate paralels with topical events, such as terrorist groups, immigration, bigotry, radicalisation in schools, war and fears of larger wars breaking out. The scale in this episode seems international, being a complex, volatile situation with lots going on, so our heroes have to split up. It is very action-packed, and often difficult to keep track of, at times a little overbearing. But I suppose you have to give the show credit for daring to try something new.