‘Invasion of the Zygons’ is the first of a two-part story and continues a story arc that began in ‘Day of the Doctor.’
The episode features a breakdown in the peace treaty established between humanity and the Zygons – a race of shape-shifting aliens that have infiltrated the human population of Earth. The Zygons, who during the classic era of Dr. Who appeared in only one Tom Baker episode, are apparently being set up to be the next big bad (or at least a faction of them are).
The treaty became established after the Doctor’s machinations effectively tampered with the minds of the humans involved and their Zygon duplicates. Neither side of the negotiating table can remember whether they are human or Zygon.
A militant faction of the Zygons is taking steps to upset the treaty. UNIT gets involved and some of the characters wind up in New Mexico.
The episode naturally ends in a cliffhanger with Clara apparently having been duplicated by a Zygon.
I found this episode oddly boring despite a few bright spots. It borrowed ideas from classic science fiction movies like ‘Invasion of the Body Snatchers’ and ‘Invaders from Mars’ without really adding much that was original. The plot twists were generally obvious. The episode’s message about partitioning between “the good ones” and “the bad ones” in a national crisis was well-intentioned but obvious.
The idea of a peace treaty being crafted by negotiators that are essentially amnesiac is a clever idea, though. Perhaps part two of the story will bring this element of the story back into the mix.