Question: what is Doctor Who for? Or more to the point, what purpose – beyond filling 45 minutes of television on a Saturday evening – does each individual episode of the series succeed in achieving?

The answer to this depends, of course, upon the nature of the episode in question. There are some episodes that exist purely for escapism, some which attempt to impart a greater truth about the nature of humanity, and some episodes which reflect particular aspects of the world around us. Some are satirical, some in deadly earnest, some frightening and some fun. Doctor Who is a series, perhaps the only series, which can change its character, its temperament – its personality – entirely, and on a weekly basis. It’s unique, there’s nothing like it anywhere else on television, and somewhere, somebody has to plot out entire runs of the thing starting with a blank piece of paper and a universe of imagination.

So this week they did Hustle in Space.

You can’t argue with the premise. Attack of the Cybermen notwithstanding, Doctor Who has never really played in the heist sandpit before – and in a series unbound by traditional notions of format, it’s a genre ripe for the plucking. After a brief preamble in which Steven Moffat reminds us of where we’re going next week, his co-author Steve Thompson – an underrated writer, perhaps better with the technical stuff than with character, but who doesn’t sell his characters short by endowing them with an excess of both quirk and personality – lands us somewhere in the middle of the plot. That’s par for the course with this kind of story, but the big question is, can Moffat and Thompson deliver on the conventions this genre demands?

There were a number of things Time Heist needed to achieve in order to be regarded as a success. Having already gathered the gang (the gathering-the-gang prologue is an easy sacrifice to make when squeezing your story into a 45 minute slot), it was paramount that we saw each one of them displaying their particular skill in service of the plan. And of course, we did; so far so-so. However, it was a nice inversion of our expectations that Saibra’s special ability, which was perhaps the more obvious of the two and thus the easiest to dispense with, later on allowed for a lovely twist that playfully undermined our expectations. It wasn’t anything spectacular, but it was thoughtful and a great example of the amount of work that went into constructing the deceptively simple plot. Every time we thought we had a handle on what was going on, and for great periods of the episode it did seem as if there really wasn’t very much going on at all beyond the standard for this sort of thing, something would happen to subvert our anticipation of the way things should be.

Many of the payoffs are unassuming and therefore rewarding in a low-key way. The use of memory worms to counteract the telepathic nature of the bank’s defence, for example, is a simple but effective confluence of inconspicuously high concept ideas, and the rewards on offer for the participants in the heist are appropriate in unexpected but obvious ways.

This brings us to the Doctor’s reward and thus the crux of the entire episode. While Psi and Saibra are remunerated with rewards that reflect both their capabilities and their anxieties, so too is the Doctor. We know there is no material prize for which the Doctor would be covetous, just as the big mystery at the heart of Time Heist is why the Doctor would be robbing a bank at all. The way these strands are tied together, along with that other riddle regarding who it was that organised the robbery in the first place, is so achingly apposite it gives the conclusion of Time Heist far more heft than could ever have been supposed at the outset. This then becomes one of those quiet, humble episodes that leaves the viewer with a warm afterglow and remains in the memory far more happily than its more ostentatious cousins often do.

Peter Capaldi is adopting the role of the Doctor with a far more natural yet still quirky charm, now that he’s had a few weeks to grow into it. The grouchiness that almost spoiled Robot of Sherwood is still in evidence, yet now it is increasingly augmented by an underlying warmth towards not just Clara, but also the supporting cast. There’s a real feeling of the Doctor we know and have loved struggling to emerge, and it’s fascinating to watch – even if the pre-titles sequence was a reminder that all this may well come crashing down in the not-too-distant future.

As for Clara, Time Heist very carefully repositions her back in the more classic companion role, after a few weeks in which she was allowed to overshadow the Doctor in some ways. His new incarnation has demonstrated on a number of occasions his capacity for being intellectually one step ahead of everyone else, and perhaps in compensation Clara has concomitantly been emotionally superior. This week, the Doctor was presented as both, and it’s nice to see him embodying the hero even as Jenna Coleman once again proves her worth to the series. Also, the dialogue and interplay between the pair is absolutely sublime

Douglas Mackinnon’s direction, sympathetic both to the actors and to the technical challenges, switches nimbly between ostentatiously appropriating genre practices, and pulling back to allow the characters room to breathe. The results are surprisingly unshowy, in spite of an assortment of diverse set pieces. It’s an intelligent reading of an intelligent script, in an episode that never once attempts to overstate itself.

Time Heist is an out of leftfield overachiever of an episode, a modest gem that will often be overlooked when the end-of-year prizes are handed out, but will never be forgotten for what it has to say about the show it so nimbly pretends not to be. It’s a beautiful example of Doctor Who trying something utterly different and re-establishing its core values as a result. There will be better episodes, and there will be more spectacular episodes, but there will be few episodes that promise apparently so little and deliver so much in return.

So that’s what even the most unpretentious episode of Doctor Who is for: to reaffirm our humanity, and to entertain us as it does so.