It's very hard not to be overly enthusiastic once a particularly exciting episode of Doctor Who airs. That adrenaline still courses through you, and you start to use all sorts of superlatives. It's the reason the Internet is often littered with people exclaiming serials to be "best Doctor Who ever!"

Let's not fight over how enjoyable The Zygon Inversion is. It's already seen by many as a 'classic.'

In fact, perhaps the only reason this serial won't be remembered as one of the best is its opening part, The Zygon Invasion which somewhat muddies the water. Quite often, it's the case that the first episode is generally considered far superior to the following parts: namely, these include An Unearthly Child, The Ark in Space, and Utopia. (Let's be clear, though: this is all so subjective because The Ark in Space is a masterpiece in its entirety, and An Unearthly Child just has a bad rep.)

Here though, I can't help but feel that if you could compress the narrative from Invasion into Inversion, this would've been the most successful two-parter of the Twelfth Doctor era.

The Zygon Inversion is very different to its preceding episode and that makes things far more interesting than if writer, Peter Harness delivered just the same. The international scope is missing, as is a main UNIT attack force, Colonel Walsh (Rebecca Front), and the rather irritating "President of the World" strand.

Instead, we get an intense personal piece that makes you genuinely care about the characters involved. And that makes this stand out as an exceptional piece of television.

We'll get it out of the way: there were two incredible performances here, and they're both from the leads. Peter Capaldi is an absolutely brilliant tortured soul, and Jenna Coleman is great, as ever, as Clara Oswald and even more captivating as Bonnie the Zygon.

Without a doubt, the Twelfth Doctor's speech about war will go down as definitive. It ranks amongst the Fourth Doctor's "indomitable" monologue and the Eleventh Doctor's stand at Stonehenge (in The Pandorica Opens) and Akhaten (The Rings of Akhaten). Actually, the Eleventh Doctor had loads of stunning speeches, including pieces in The Eleventh Hour, Vincent and the Doctor, The God Complex, and The Time of the Doctor. Bizarrely, though, we weren't given an iconic Peter Capaldi speech in Series 8 – the closest we came was in Flatline – so frankly, it's about time!

He shows a tempered anger and anguish as he explains that, "When you fire that first shot, no matter how right you feel, you have no idea who's going to die. You don't know whose children are going to scream and burn; how many hearts will be broken; how many lives shattered! How much blood will spill until everybody does what they're always going to have to do from the very beginning: sit down and talk."

And finally, we get an allusion to The Day of the Doctor that feels genuinely right. The Doctor was once going to press another button and wipe out all of his own kind. The thing that stopped him then was Clara – and this Zygon now wears her face. In a beautiful inversion of that scene from the 50^th anniversary, he has to beg her to change her mind.

Except, if you know Doctor Who at all, you'd have realised very early on that the boxes were always going to be empty. The Doctor would never hand any race the capability to wipe out everything and everyone.

Remarkably, that doesn’t matter: it remains a startlingly intense scene, one with real peril.

Peter Capaldi has spoken in the past about how he initially found acting opposite the Daleks difficult because they don't offer a conventional eyeline. It was, then, a genius move to have him play off Jenna Coleman, who has never been anything less than fantastic.

It's immensely pleasing to see an 'evil' version of Clara. She's unsettling, stunning, and affecting. Jenna really gave this her all – which makes it even sadder to think she'll be leaving imminently.

It's not fair, however, just to highlight these two actors. Let's not forget Jemma Redgrave as Kate Stewart, who has become such an important part in Doctor Who so easily, it's astonishing. For her, too, the scene in the Black Archive is memorable, one of her best so far. In one episode, she goes from quoting her father – "five rounds rapid" – to apologising to the Doctor.

Sure, she'll return to her gun-toting ways, thanks to a handy memory-wipe, but this was a touching nuance to a character that could've just been a mere soldier. It's reminiscent of The Power of Three where she was witty and intelligent. Since that, she's been diminished, but at least there's a glimmer of hope again.

Ingrid Oliver also puts in a solid performance and Osgood's return feels justified now; last week, it simply felt shoe-horned in, but the very idea of the Osgood Box (well, boxes) is smart and feels true to both her and the Doctor.

Osgood will return once more, I'm sure, but fingers crossed it won't be a further opportunity for a Zygon story. I'd like to see her as an active member of UNIT, not just a cosplaying fan of the Doctor.

There's not a huge amount of narrative in The Zygon Inversion, but that's certainly not to say it was an insubstantial story. The dialogue is where this tale excels, and no, not solely in that scene in the Black Archive. That one does tend to swell in your memory to shadow other excellent plot strands, namely the claustrophobic dream sequence with Clara (a la Last Christmas), and that sole Zygon, Etoine (Nicholas Asbury) forcibly being revealed living among his human neighbours.

There's a wonderful juxtaposition there: body horror as the true alien emerges but real sadness in his rhetorical "I never wanted to fight anyone; I just wanted to live here. Why can't I just live?" Still, Bonnie's plan seems half-baked at best: without what the Osgood Box supposedly can do, does she really intend to personally visit 20 million of her own kind in order to force them out into the open?

It's all perfectly lit and the direction by Daniel Nettheim is ideal for this confined story with a huge scale. Nettheim is more than capable of presenting atmospheric scares alongside international environs: he's got a great sense of what Doctor Who actually is and how it can look. Here's hoping he's invited back for Series 10.

Niggles persist – the biggest being that it's the Doctor's former companion, Harry Sullivan (a personal favourite) who created the Z67 gas. It's simply not something you can ever envisage Harry doing.

Nonetheless, it is hard not to be overly enthusiastic about The Zygon Inversion because it has so much going for it. Great performances, great dialogue, great direction: these all combine to make truly great Doctor Who.