Back on the Episode Day of the Doctor, the Tenth, Eleventh and War Doctors were able to create a peace treaty between the humans and the Zygons. The lynchpin was the two Osgoods. One died and that has led to the possible failure of the treaty. The episode centers on a splinter group that is not happy with hiding what they are, instead of desiring to share the earth, they want it all. Osgood starts this ball rolling by sending the Doctor a distress signal.
As various investigations happen we discover the fates of many characters. The episode ends with it looking dire for the Doctor. The theme of this episode is coexistence. How can two groups coexist if one is not allowed to be who and what they are. A question that it seems is being asked is how much assimilation is too much. Zygons are shapeshifters so that is even a trickier question. Especially in order to fit in they are denying their true shape and heritage.
The new faction it seems is destroying all those in the leadership that were pro-assimilation. They now want to take their new home away from the original inhabitants. This episode seems to be a parallel with themes about immigration and colonialism. The Zygons are like many European-Colonial cultures that wanted to usurp the indigenous peoples. This episode also deals with certain military strategies when dealing with extremists belonging to a minority. UNIT wants to attack, but the Doctor sees that and attack would mobilize some of their Zygon allies into Zygon enemies.
Using a science fiction to ask the tough cultural ethic questions is nothing new. It has been used so we take the parts of the controversial subject and divorce it from many of the things that we have vested interest. We are then able to re-think our positions. That is what good Science Fiction does. I love it when fiction and drama makes me think and this episode did. I give it a B+.