An immortal girl. An alien artefact called the Eye of Hades. Notorious highwaymen. This had all the ingredients to become an instant classic.

Sadly, there was just something missing and resulted in a story that was – dare I say it? – a little dull.

There were a lot of good things here too, but the pace of the narrative just overshadowed a number of brilliant elements. Let's start with the writing. I like Catherine Tregenna. She wrote some of the most notable episodes of Torchwood, including the vicious Meat, the wonderfully-creepy Adam, and the absolutely beautiful Out of Time, surely a contender for the stand-out story of Series 1. Even though showrunner, Steven Moffat had to coax her into working on Doctor Who, I had high hopes.

The story was fine: not as subtle as I expected but Tregenna adapts well to a character and notion thrust upon her. Nonetheless, she didn't have to study Ashildr (Maisie Williams) in The Girl Who Died – her personality is a stark contrast to the hopeful storyteller of last week. That sounds like a criticism and true, I was initially skeptical of that change.

It makes sense, however. As we briefly glimpse snippets of Ashildr's now-immortal life, you can understand why she's altered so greatly. I prefer the intelligent, bright-eyed girl who stood up to the Mire, but 'Me', as she now calls herself, is nevertheless an intriguing person with plenty of traits and history to explore.

Unfortunately, that's not exactly what we got. Again, we were presented with snippets of life, almost egged on by an oddly-whimsical soundtrack. The tone didn't fit, as if these events – the Black Death in 1348, the Battle of Agincourt, a bout of scarlet fever – were mere comedy sketches, bookended by Me's lamenting her own life and the ignorance of those around her.

It means that while Williams plays the anguish convincingly, the audience feels somewhat removed from the emotion as her character loses her children. (Anyone questioning why she didn't use her spare medical kit to turn one of her babies immortal may not realise they would stay the same age forever, just as Ashildr/Me has, that she'd have to choose which to save, and that she wouldn't wish immortality on anyone, let alone her own kids.)

Still, there's real sadness for her predicament and the Doctor's realisation of the implications of his prior actions. There's a selfishness about him: he saved her life because he was essentially responsible for her death; furthermore, he won't take her with him. Even if the latter is so that they keep a sense of perspective, it's very harsh on Me. Plus, it's clear she does lose perspective, so her presence would only hamper him.

That's not a bad thing: we should be reminded of the Doctor's alienness and his long life becoming a burden. It's a theme we've pondered frequently in the past, but the Doctor's one of the sole characters on television who can properly explore the notion, so of course it's right that there should be a bit of brooding.

The Woman Who Lived, as a whole, is a brooding piece, however. Once the closing credits come, we're left in no doubt that this episode is just a prime example of foreshadowing. We'll see Ashildr again. She'll keep an eye on the Doctor. And Clara will go the same way as all the rest.

Jenna Coleman's absence is an odd move, considering this is the last run of stories to feature the companion. Perhaps it's to give us a taste of the Twelfth Doctor on his own – or at least becoming a sort-of companion himself. However, it doesn't entirely work. Without a secondary lead figure, each scene just plays out, not intersected with any further peril or, indeed, perspective. Splitting the action keeps a piece alive and driving towards a conclusion where different plot strands meet. This was a simple affair – again, not necessarily a negative thing, but the pace suffers considerably.

Rewatching the episode, unburdened by expectation, is a more pleasant experience. You're not waiting for a distraction from the Doctor and Me discussing the duration of their lives. Still, there's not enough incident or horror.

Leandro (Ariyon Bakare) is an old-school antagonist, duping Me into basically giving him the keys to the kingdom. He's not an effective enough threat, though, staying in the shadows for much of the drama and growling, snarling, and presumably prowling – fairly uselessly, it has to be said – in his final scene. This wasn't particularly anyone's fault, but is by necessity of being a sub-plot.

The dynamics between the Doctor and Me were central; there's no doubt about that. Both Peter Capaldi and Maisie Williams react to one another perfectly, helped along by the truly sparkling dialogue.

The pair speak with poetic fluidity, their rapport contrasting greatly to the 'dumb' conversation of those around them. No one else is really given enough to do to expand their characters and their motivations, yet it's sufficient in highlighting how the Doctor and Me almost exist on a different plane. Real people might not speak with such elegiac rhythm, but these two aren't like us.

Comparisons to smoke and mayflies are particularly noteworthy for their elegance.

The script wasn't completely without humour though: Sam Swift (Rufus Hound)gets a lot of witty dialogue (and naturally, it's delivered perfectly), and it's pleasing to see that Series 9 writers have remembered that Capaldi made his name on a comedy show, so yes, he can deliver comic material. I especially enjoyed his realising why he had named his curioscanner as such.

Ed Bazalgette, director, has more to work with this week than last: before, he was confined largely to Ashildr's village, but this time, he gets different locales both geologically and in relation to time. Things are grander here, not only in the quarters of Lady Me but also in the house she and the Doctor go to rob.

Nonetheless, there's a homely charm to its limited cast, and pleasing mirroring when a village becomes the harbinger of danger towards the tale's conclusion.

Everything seems a tad too dark, however. Yes, it still looks stunning, but its lack of illumination at times makes for a gloomy story, both physically and figuratively. This is the very essence of a brooding piece.

The Woman Who Lived isn't a success; nor is it a failure. It's simply a story whose parts are better than its sum.

Images: BBC.