This is the first episode this series to feature truly effective villains. The Zygon antagonists feel like a match for the Doctor and not just a convenient plot device. Their story is interwoven through every part of the episode and we are never unaware of their influence. They have a credible and intricate plan of infiltration into the power structures of human society that is undertaken over time and carried out with finesse and intelligence. This is how you write a bad guy. There are many levels of treachery in this tale of cultural unrest. The Zygon splinter group feels betrayed by a preceding generation that agreed to a peace treaty that effectively forces them to give up their heritage in exchange for a masquerade that is designed to prevent panic among the human population. Sandeep and Hitchley are deceived by people who appear as their parents. The fear of a loved one turning against you is a primal one that most humans share and this fact makes the heartbreaking scene between Hitchley and his supposed mother eerily effective.