There are two important plot-points from previous episodes that need to be remembered going in to this latest episode of Doctor Who. The first concerns the 50th Anniversary special episode, The Day of the Doctor, in which the simple joy of having Matt Smith and David Tennant appear alongside John Hurt as a, at that point, so far unrevealed incarnation of the Doctor was somewhat wasted on a goofy plot involving shape-shifting aliens (not that there was any real problem with the aliens, themselves, of course - it just didn't seem like something that required three Doctors). The end result of this episode was a peace treaty between these shape-shifting aliens, the Zygons, and the human race - a peace treaty that, essentially, allowed the Zygons to integrate with the human race and live out the rest of their lives as humans.
The second important plot-point concerns the character, Osgood (Ingrid Oliver). By the end of the anniversary special, Osgood was left on relatively good terms with her own Zygon double. Also, by the end of the previous season of Doctor Who, Osgood was murdered by Missy - though, of course, whether it was actually her, or her alien double who had died.
As the episode opens, though, the peace treaty that has existed for the past few years seems to be breaking apart - as a small faction of the 20 million Zygons spread over the planet turn against both the human race, and those of their own kind who they have come to regard as traitors. The main issue, for this small group of Zygons, is that they have come to resent the fact that they are being forced to live in human form - they want to be free to live on their own, natural forms. But, it's not just that - they also want a world of their own and, still lacking any other options, they have decided that Zygons were right the first time, and that they should just take Earth.
The Doctor is brought back to Earth when he receives a message of Osgood - a desperate plea for help in the face of this 'nightmare scenario'. But, which Osgood is it, though? Well, as she says herself at one point in the episode, it doesn't actually matter. By refusing to reveal, to anyone other than themselves, who is the human and who is the alien, the two Osgoods have set themselves up as a symbol of the peace treaty, itself. One may be gone, but the other is adamant that what they had tried to stand for still has value - even as this radical faction of Zygons continue to consolidate their power. It's definitely a fascinating idea - and, a great use of the character. I was never so attached to Osgood that her death in the previous season bothered me as much as it seems to have some other fans - but, I am still glad to see her used in such a creative and important way, here.
The main problem faced by the Doctor, though, is the increasingly unlikely possibility of keeping the peace between the two races. The Doctor is worried that a direct attack on these aggressive Zygons would only escalate an already dangerous situation, as he believes it will only result in the radicalisation of more of the species. Yet, his efforts to engage in peaceful talks is undermined by the fact that these already radical Zygons just don't seem interested in talking. Also, their natural shape-shifting ability seems to have improved significantly in their time living among humans - making them much more dangerous, and much harder to detect.
This is a fantastic set-up for a tense and unpredictable Doctor Who story-line - yet, unfortunately, the episode doesn't seem to make the most of its potential. Some moments of attempted drama simple don't work as well as they were, clearly, supposed to. One in particular, involving a soldiers refusal to shoot a Zygon who has taken on the form of his mother, is clearly intended to be dramatic - but, instead, it comes across as contrived. Also, with three separate plot-lines going at once the episode, as a whole, often feels a bit unfocused. We have the Doctors efforts to keep the peace in his role as 'president of the world' (I'd completely forgotten that this was a thing that actually happened), then there's Clara's efforts to uncover the Zygon threat in London - and, to top it off, there's also Kate Lethbridge-Stewart's (Jemma Redgrave) investigation in New Mexico. In an already complicated episode, having three plot-lines taking place at once just feels like too much.
The most distracting aspect for me, though, is probably the Zygons, themselves. While disguised as human, their potential for causing tense paranoia is undeniable - but, in their natural form, they look ridiculous. Doctor Who has managed to come up with some wonderfully effective designs for a variety of genuinely intimidating alien threats over the years - but, the Zygons simply aren't one of them. Scenes which include Zygons still disguised as humans can be effectively tense, of course - but, the moment they change to their natural forms, all tension is lost.
The real difficulty of an episode like this, though, comes from its use of outlandish analogies for real-world issues. Whenever you have something like this in science fiction or fantasy, there is always the issue of exactly the fictional situation is going to be resolved - and, whether that resolution is going to be intended as any sort of statement from the creators. With the current Zygon situation, we have aspects of both a refugee crises and the issue of terrorism - and, so far, those moments were the show seems to be drawing direct attention to the real-world parallels have come across as uncomfortably heavy-handed. With this being only the first half of a two-part story, though, we are going to have to wait until next week to see how any of this is actually resolved. It is, honestly, something that I am both interested to see, and vaguely worried by.
After a great season, and the fantastic previous two-part story, this episode probably marks the first real disappointment of the ninth season of Doctor Who. Despite its issues, though, the episode still manages to set up a fascinating and difficult challenge for the Doctor. Regardless of my own, personal, reservations about how this episode played out, I am still interested in seeing where it all leads next week.