★★★★★ Eccellente! Once in a while a Doctor Who story comes along like no other you’ve seen before. One that keeps surprising, and amusing, and tantalising from start to finish. 

Extremis is one of those stories. More accurately, two of those. It only fully makes sense at the end, and then – if your head isn’t hurting too much – it’s hard to resist the impulse to go back and watch again from the start. 

It’s ingenious. You might get a long way into Extremis before realising it’s telling two largely unrelated narratives. And that neither is happening in the here and now. The first (Missy’s execution) has happened “A Long Time Ago”. The second is unfolding inside an alien simulation of the present. 

Only snippets take place in the Doctor’s present “reality”: the early section marked “Today” down by the vault, and that dissolves with a crackle (either side of the title sequence) to indicate we’re leaving reality; then the final moments after the Doctor receives the Extremis email from his virtual-reality counterpart.

At the halfway point of the season, Steven Moffat uses this unusual structure to shift things up a gear – revealing some of his secrets and setting out his stall for the weeks to come.  

The Long Time Ago story finally tells us how Nardole came back into the Doctor’s life and who is in that bloomin’ vault. Surely only the slowest-witted had much doubt it was Missy. (To be fair, this RT slow-wit had pictured John Simm’s Master inside too.) But no more farting about. We’re told pretty much straight up it’s her. It’s explained (kind of) how the Doctor swore an oath to guard this “quantum fold chamber” for a thousand years and why he’s seemed a less-than-strict captor. 

The execution scenario messes with us for a while. It plays as if the Doctor’s life is on the line and Missy is his executioner – before deftly switching. We don’t know why Missy’s number is up, but we do know there’s no way her best frenemy would actually destroy her. 

To my delight, even after all these years, the writers (and for now it’s Steven Moffat) are finding new ways to play with the enmity and close friendship between the Doctor and the Master. This dynamic goes back to their childhood on Gallifrey and, in TV terms, to Jon Pertwee and Roger Delgado in 1971. That was part of the magic formula (alongside Nick Courtney’s Brigadier and Katy Manning’s Jo Grant) that captivated me as a nipper. So I’m a sucker for any retread into that territory. 

The “execution” scenes are a gift for Michelle Gomez, a scene-stealer who puts the “arch” into archenemy. We see new shades to her malevolent Mary Poppins shtick: she acts cowed, is almost sympathetic. Always funny. Basically, no matter how naughty she is, you could imagine having more fun with her than you would with the Doctor. Something Jon Pertwee feared all those years ago.  

And this is the simpler storyline to follow, remarkably. What really keeps you on your toes is the Veritas thread. The business with the Vatican’s sombre cardinals calling on the Doctor, the visit to the Haereticum, their library of blasphemy, the forbidden text that makes all those who read it kill themselves… Bill may say it’s all very Harry Potter, but it’s Umberto Eco’s The Name of the Rose for me. 

And the Pope? Hilarious! I choked with laughter when the Pope waltzes into Bill’s apartment interrupting her date with Penny. “Do not put the Pope in my bedroom!” bridles Bill. Steven Moffat is having some fun, while maintaining a level of respect for the Catholic Church. It’s not all that bizarre seeing His Holiness and his cardinals inside the Tardis when you consider how closely the Time Lords in all their regalia were modelled on the Vatican. 

I began by saying Extremis is like no other Doctor Who story. You want the Veritas? I did not speak the truth. A massive library that merges with a virtual-reality environment was very much the premise of Steven Moffat’s brilliant 2008 two-parter, Silence in the Library/Forest of the Dead. You could also argue that the disfigured Monks and their invasion by stealth are rather similar to the Silence who operated just beyond human perception and plagued Matt Smith’s Doctor. But these Monks are bloody hideous. Barely alive. Their mouths twitching, not quite matching their speech – rather like the original Cybermen. (Voicing the Monks is Tim Bentinck, aka David Archer in Radio 4’s The Archers.)

Extremis suddenly becomes traditionally, gloriously Doctor Who-y when the blind Doctor (Capaldi, magnificent) remains in the Haereticum “reading cage” to fiddle with some gadgetry, while his two friends wander off and get into trouble like companion duos of yore. In an era that gives the sidekicks a lot more credit, Bill and Nardole stumble upon the truth before the Doctor. 

In a stinging twist they uncover the Veritas and must face the fact that they aren’t real. This works so well because Bill and Nardole are cartoonish; they look like they’ve been storyboarded, sketched, inked in – then sprung into life by the charm and personality of Pearl Mackie and Matt Lucas. After Nardole shimmers into oblivion, it’s horribly tense waiting for the inevitable confirmation that, yes, neither Bill nor the Doctor are real. 

It’s for the unreal Doctor to explain (gravely intoned by Capaldi): “Imagine an alien lifeform of immense power and sophistication, and it wants to conquer the Earth. So it runs a simulation, a holographic simulation of all of Earth’s history and every person alive on the surface. A practice Earth to assess the abilities of the resident population, especially the ones smart enough to realise that they are just simulants. Inside a great big computer game… Those deaths, they weren’t suicides, those were people escaping. It’s like Super Mario figuring out what’s going on and deleting himself from the game.” Smart twists that turn the episode on its head. 

As the 45-minute window starts to close, you sense there’s a lot more to say. And yes, this is only a chapter in a page-turner with threads that will be picked up next week and in weeks to come. 

But what of Missy? The Doctor confides in her, yet she stays firmly within the vault. Let her out now! May she join in as many of Capaldi’s final days as possible! I can tell you now that we won’t be seeing her next week, but the word is out that she’s in episode eight, The Lie of the Land, long before what had been her next billed appearance, episode 11. 

Sure, this brand of Who doesn’t float everybody’s boat. But, really…? This is such confident, breath-taking television from Steven Moffat. Peter Capaldi is totally commanding – a strong but nuanced performance week after week. And because I can create my own simulation of the future, I’m missing them already. 

Steven Moffat's talks in depth to Radio Times about Extremis