The events of the Day of the Doctor lead us to this point, Earth home to humans and Zygons alike....
Twenty million Zygons rehoused on Earth as part of UNIT's operation double. All that's needed is one rogue Zygon to upset the balance and break the ceasefire.
I always like references to previous episodes, a few to enjoy here, the Day of the Doctor, Death in Heaven, Terror of the Zygons. Absolutely loved the reference to the question marks, and the underwear, brilliant! The benefits line loved that too.
I just love Osgood, she's a brilliant character, they realised her popularity, I'm sure that's why they killed her off originally. Good also to see Clara back on form too, best outing for her for a little while. So good to see more from Jaye Griffiths, a more meaty part then she had in the Season opener, she's been off our screens for too long.
I'll be honest I wasn't sure about the re-design of the Zygons in the Day of the Doctor, but here they just looked so effective. The 70's Zygons were a design classic.
A genuinely creepy episode, I absolutely loved it, just brilliant. You watched the episode and genuinely didn't know who was a Zygon and who wasn't. The tension was expertly created. Great development to the Zygon race itself, we get to learn a bit more.