"You must not watch this."

That's a deceptively clever opening line. Because whenever someone says that, the thing they're trying to warn you off suddenly becomes immediately compelling, certainly to the point where you have to watch it. In fact, Sleep No More relies heavily on you keeping your eyes wide open and paying close attention.

Yep, that's just what everyone wants.

It's a major bug-bear of mine that people complain they don't understand what's going on, but don't actually pay attention. They're too distracted by social media or the phone going off. Doctor Who, however, is something that you really need to keep an eye on; dedicated fans especially won't turn away from the screen. Gagan Rassmussen (Reece Shearsmith) – and thus writer, Mark Gatiss – is betting on this.

Sleep No More is a whole different layer of 'meta,' not content with blurting out "Doctor Who?" at given opportunities or wishing all of us at home a very merry Christmas. Because the whole point of this episode is... the episode. That, in itself, is the monster, and on repeat viewings, we're still not party to what's 'real' and what's not.

That's a fascinating conceit, giving a solid reason for the experimental nature of the story's presentation. Why is it a found footage serial? Because that's the whole point.

Thanks to a lack of proper titles and soundtrack, you really feel like this was salvaged and immediately aired. The makeshift title was a welcome one-off change, smartly executed, while the absence of Murray Gold's typically-wonderful tracks puts you on the backfoot. Gold's greatly experienced with adding layers of tension to tales, but here, you still get that effect because it feels more true to life.

It's not entirely without music, of course, and Mr. Sandman is a useful plot point throughout. Actually, it's a great representative of the Morpheus creature: you'll be singing this infectious song long after the credits roll. There's also something eerie about the rendition – that only an earworm like that, first recorded in 1954, could still crop up in the 38^th Century and so completely at odds with the environment. The La Verrier Space Station is now a dark, grim place to be and those cheery pop singers juxtapose with that and the supposedly cosy Morpheus Machine. It blurs the definition of dreams and nightmares.

Ah yes, an earworm. An argument could be made that there's a link between this episode and Under the Lake/ Before the Flood, just a few episodes ago. I loved the two-parter, but this arguably handles the notion more deftly. Instead of carving the alien symbols on the minds of the Doctor (Peter Capaldi), Clara Oswald (Jenna Coleman), and co., the electrical connections are forced onto the viewer.

Although there are many 'firsts' for Doctor Who in this episode, it nonetheless alludes to various other stories and definitely has a similar tone to adventures like The Impossible Planet/ The Satan Pit, 42, and even Image of the Fendahl. Sleep No More references Frontios (and I love Frontios!) and Doctor Who and the Silurians, once again bringing up Homo Reptilia's naming problem.

I can't help but feel that its close cousins are The Edge of Destruction and Midnight. Both encapsulate the experimental nature of the show, sometimes by necessity and sometimes as the main purpose of the plot. The two stories are poles apart, but the restrictions make them divisive and massively interesting.

That's Doctor Who down to a tee. It's why the show is celebrating its 52^nd birthday very soon.

(Similarly, audio adventures like Whispers of Terror and Dead Air also utilise their own format well, both incredibly effectively. If you've not heard them, you need to.)

The Edge of Destruction, Midnight, Sleep No More: these are stories that enrich the series and showcase how malleable the format is; they will never become the norm but they still needed telling. They're important.

All three feel rather restricted – in a positive way. Last series, I got annoyed by Deep Breath and, to a slightly lesser extent, Into the Dalek because they felt like they needed to be widescreen movie-esque pieces but instead their visuals were oddly stunted. In Sleep No More, that's the whole point.

If you're well-versed in Who, you'd have likely picked up the impossible camera angles, particularly those from Clara's POV. Viewer immediately presume the rescue crew have headcams, and it's only when Nagata (Elaine Tan) says they don't have those, that the implications start to sink in.

Director, Justin Molotnikov should be applauded for such a stunningly-realised episode. His work is a real pleasure to behold; every shot has really been agonised over. The bold, striking visuals are reminiscent of Silence in the Library/ The Forest of the Dead, The Bells of St. John, Cold War, and even stories as far back as the first episode of The Sensorites.

Steven Moffat, showrunner, has previously said that every new writer and director needs to mould Doctor Who, to make it their own, and Molotnikov does this expertly. Sleep No More is a masterclass in first-person storytelling.

And yet it's not wholly satisfying. While the rescue crew generally feel real, the nature of 474 (Bethany Black) leaves a lot unexplored. Maybe we'll come back to the Grunts in future – there's definitely something interesting about the concept – but for now, she seems somewhat undercooked. It might've tied into the wider Whoniverse a little better if she were Flesh, for instance...

Another point of confusion is its conclusion. Sometimes, the Doctor loses. Fine. Good, even! We can't have a perfect hero. But the structure does leave something to be desired. Can you imagine the Doctor really just disappearing in the TARDIS and letting the wider issues resolve themselves?

Like so many experimental episodes, I do wonder what the casual viewer would make of it. The show shouldn't shy away from being edgy and decidedly different in favour of a typical Monster of the Week drama simply to satisfy the masses. Still, the narrative would seem awfully segmented if you're not one to rewatch the story in light of Rassmussen's final admission.

These problems make the story extremely divisive: one group will no doubt call it a terrible, dull, and ultimately unenjoyable story, while the others would call it fantastic art. In case you're wondering, I'm part of the latter group.

The biggest shame is that Sleep No More doesn't give Clara enough to do. Seen as it's looking increasingly likely that next week's Face the Raven will see her leave the TARDIS, it really does taint this episode. Even if she stays until Series 9's end, we still don't have much longer with Ms. Oswald.

Please do excuse me. I think I've something in my eye.