Part ones are arguably far easier than concluding part twos. Creating mysteries, locked box conundrums, impossible life-or-death situations is far, far easier than providing solutions which manage to be simultaneously surprising, satisfying, and with hindsight seem inevitable. Recent Doctor Who is (understandably) littered with examples of part twos which fail to live up to the promise of part one, and sometimes even tarnish the memory of part one.

A notable exception is last year’s The Zygon Inversion, a co-pro between showrunner Steven Moffat and Wallander scribe Peter Harness, the conclusion of which is still for me a high water mark for the series as a whole (up there with Human Nature, Dalek and, yes, I suppose, Blink).

I didn’t really review Extremis last week, but suffice to say I thought it was a fairly empty and meaningless exercise. A glorified “and it was all a dream” ending which makes very little sense on any level. So, I had rather mixed feelings sitting down to watch this week’s installment.

I needn’t have worried. This is masterly stuff, playing to both writers’ strengths. I suspect Moffat’s hand in the chain-of-chance plotting which leads lab workers Rachel Denning and Tony Gardner to accidentally create a bacterium antithetical to all life. And Harness’s contribution I imagine is likely to be the stunning pair of moral dilemmas – first those faced by the three generals, and then the even greater one faced by Bill at the very end.

It’s also very well worth pointing out that – for perhaps the first time since Matt Smith took over – this is serialised storytelling done right. Doctor Who 1963-1996 was always fundamentally a serialised anthology series. Even linked seasons like The Key to Time or (gawdelpus) The Trial of a Time Lord fell neatly into self-contained sections. Parts two, three and four might be a little hard to follow if you hadn’t seen last week’s episode, but every part one was a new story, and all you needed to know was the Doctor travels in time in a police box and you could start watching.

But in 2005, the television landscape was very different. Post Babylon 5, post Murder One, post The Wire, audiences were happy with – maybe even expected – a series arc at the very least. Russell’s approach to this was cautious. Mentions of Bad Wolf, Torchwood or Mr Saxon could be picked up by devoted watchers, safely ignored by casual viewers.

When Moffat and Smith took over in 2010, we were post Breaking Bad, post Man Men. And Moffat was keen to show that Doctor Who could compete. However, not wanting to sacrifice variety, the end result was a pretty ghastly muddle at times, with “arc” stories rubbing up against “non-arc stories” sometimes in the clumsiest of ways (see Night Terrors for arguably the worst offender in this regard).

Under Capaldi’s reign, things have been a bit smoother, with stand-alone-stories generally being the order of the day, but we’ve still had to suffer end-of-season gibberish like Death in Heaven. Now it seems like the balance between these two forces is being struck perfectly. Both this episode and the preceding one stand alone, but they work better together. In fact, Pyramid retrospectively flatters the earlier episode. I’m considering bumping it up from two stars to three.

Back to the episode itself. The structure is more sophisticated than, say, Robot, but far simpler and far less OCD than many recent stories. The Doctor madly scrambles to figure out what the monks in the pyramid are up to, but we know he’s looking in the wrong direction. Finally, the Doctor’s blindness pays off – he figures out how to find the source of the impending catastrophe.

Here the physical geography of the lab is a little confusing. It would have been better to have found some way in the scripting or the shooting to clearly demarcate which areas were compromised and which were safe, but the key elements of the problem are assembled very neatly. The Doctor on one side of a door. The TARDIS on the other. Nardole, incapacitated inside. And the Doctor’s sonic glasses totally unable to read the numbers on the combination lock. As a piece of plotting, forcing Bill to sacrifice the stewardship of the Earth out of pure love, it’s basically perfect.

Let’s have a little talk about that prop though. The actual combination lock itself looks like a child’s toy and no lab in the world ever had a combination lock like that. Surely it should be a keypad? That’s a problem if Denning knows the code, because then the Doctor can enter it without looking at it, but surely a better solution would be to have the code be 10-12 digits long – so long that Denning and Gardner keep it written down. Now Denning doesn’t know it, and any sighted person would be able to read it and we don’t have to have that ridiculous looking Duplo prop.

This is a minor niggle, and I hate giving episode five stars when there’s a conclusion still to come, but this is really, really good stuff. The regulars are on great form, the UN quartet do everything that’s asked of them, the effects are all top notch and director Daniel Nettheim generally keeps things moving. I’m going to keep half a star in my back pocket though. 4½ stars and onwards to The Lie of the Land.

So... what did I think of Extremis?
So… what did I think of The Lie of the Land?