At the end of the last episode we found out Clara had been replaced by Zygon Clara. In this one, we discover Clara has used her link to the Zygon Clara to help the Doctor, even though it appears he died. Of course he survived. Zygon Clara is trying to find the Osgood box while Clara is using her connection to cause issues. There are two boxes one for humans and one for Zygons. There are two buttons on each box. The human box has one button destroys the Zygons with an agent that turns them inside out and the second destroys all of London. The Zygon box has one button the reveals all Zygons’ true form and the second freezes them in human form permanently. The identity of which button does which in not known. It is not clear even if Osgood or the Doctor know; they are however labeled Truth and Consequences.
The themes in this episode are how horrible war is even if justified, the mutual destruction option is also not a true option, and that for every revolution there is always the threat of another. It is implied that the boxes may be empty. There is the question: is that true? One never really knows with the Doctor. The Doctor regardless puts both races’ leaders in an impossible decision. They want to end the hostilities and win, but with 50/50 odds it is possibly a no win situation
Zygon Clara at one point forces a Zygon, who is happy with the status quo, to be outed as a Zygon. When the Doctor told the outed Zygon he was on his side, the Zygon states he had no side and refuses to take one. This resonates that extremist groups try to force people to choose their side. Basically not everyone in the Zygon extremist uprising is a willing participant. The extremists are trying to force their point of view on their entire race.
The show examines the futility of war, that it forces behavior that is reprehensible. The Doctor points out that he too was pushed into the idea of possible genocide of his own race. This continues the idea of the Doctor haunted by his most extreme decisions, even ones he avoided. He loathes the fact he came close to doing things that are antithetical to his code of honor. Hew is working so that no one ever has to make those kind of choices. Overall it is a deep and thought provoking episode, definitely a B+.