Back in The Witch's Familiar Davros talked about a Gallifreyan legend of a hybrid. The Doctor did not react well to this. This was in an episode in which Davros had tried to create half-Dalek, half-Time Lord hybrids (y'know, because Davros's defining characteristic in the new series is that he comes up with this kind of mental plans). That didn't work, so we were left to mull over the topic of the hybrid.
Thankfully it's not Steven Moffat's take on the half human nonsense from McGann's TV Movie. That's something that seemed like a worrying possibility for a while. It would have been spectacularly poor. I mean season twenty-two levels of poor. Thankfully The Woman Who Lived seemed to make it clear that the hybrid in question was not, in fact, the Doctor or a new breed of Dalek or something else based on years-old continuity. It was Ashildr from The Girl Who Died.
At the end of The Girl Who Died Ashildr was made immortal using a modified bit of tech from enemies of the week the Mire. That made her a human-Mire hybrid. This episode made it clear that she is to be taken as A Problem™ for the Doctor. Which could have been boring. Thanks to an engaging script from Catherine Tregenna, excellent work from Peter Capaldi and Maisie Williams (as well as, perhaps bizarrely, Rufus Hound), and the decision not to try and squeeze Clara into the episode it wasn't. It was a well-judged piece of writing that included just enough of a plot (an invasion orchestrated by a duplicitous, fire-breathing space lion wearing a metal headband) to keep things interesting in between stretches of what the episode was really about, the Doctor confronting his decision to make someone immortal.
Tregenna pitched things perfectly. After starting out with an amusing sequence in which the Doctor stumbled into a heist she gave us around fifteen minutes of the Doctor and "Me", the character having abandoned the named Ashildr centuries earlier, discussing their differing experiences of living lives so long that they are forced to watch loved ones grow old and die. Or, perhaps, their ability to forget and repress these experiences. These scenes were deftly handled, avoiding heavy-handedness but carrying the emotional heft required to make them work. It became clear across the course of the episode that Ashildr or Me or The Knightmare (srsly, pick a name, guys) was less unhappy with the "curse of immortality" (good because that trope is played out) and more the fact that the Doctor has lumbered her with the slow path through history, forcing to live every day in full and in order as he grants himself the freedom of time and space. Implicit in this was the unspoken accusation that the Doctor has made it easier for himself and isn't interested in lessening her burden. 'You trapped me in my life!' exclaims Me at one point. It's a good line but it would be more accurate to state that the Doctor's trapped her in history.
Me clearly isn't framed as a baddie in the style of Missy or Davros. She's given a great deal of sympathy. Meanwhile much is made of the Doctor's poor judgment and while it's not dwelt on there's that callous refusal to take her into the future and let her live the life she wants. His inaction is forcing her to lead a life she doesn't want and can't escape and was trapped in by him. But we are also left in no doubt that this is a woman who could become terrifyingly bad. Her willingness to kill is shown several times, most notably when she's keen to shoot her way out of a burglary job and when she sacrifices Sam Swift to open the rift she needs to escape the planet. It's a disregard for life that's a staple of recurring villains in the show.
This is clearly something Moffat has been moving towards for a while, the Doctor creating his own enemy. Had he handled it himself I don't think it would have been as good as it has been across these last two episodes. But in handing the job over to Catherine Tregenna and Jamie Mathieson he's let writers who will focus on the character, rather than the idea of the character, come to the fore. This is how he should run the show all the time. The closing flourish of Clara turning up to show the Doctor a selfie with Me (or whatever modern pseudonym she's taken) looking on pointedly from the background is clearly an addition he would have added or requested but the bulk of the episode feels unlike something he'd have written himself. This is for the better, because Moffat's authorial voice has been oppressive at points over the last five years.
That closing shot gives us a tantalising glimpse of where this series will end up. With Jenna Coleman's departure confirmed it seems safe to assume that the modern day incarnation of Me will be responsible for her death, talking as she did about how she'd keep an eye on the Doctor and exhibiting jealousy at the fact that Clara was taken with him on his continued journey. That would tie into the theme of the Doctor creating his own enemy in The Girl Who Died and The Woman Who Lived and the debate about compassion in the Doctor and Davros scenes from The Witch's Familiar. Everything points towards Clara dying as a result of Me's actions, forcing the Doctor to admit that his act of compassion in saving her was ultimately wrong. There will be more to it than that, of course, but it's looking like the richest season arc of the Moffat era right now, and if it turns out to be true this episode will be a pivotal part of it.
All this plus a reference to the Terileleptils. What's not to like?