Full disclosure: to me, Toby Whithouse is one of Doctor Who's strongest regular writers. I've enjoyed all his Who work, but The God Complex (2011) is a particular favourite. As such, this two-parter has been my most anticipated story of Series 9.

Fortunately, this sole episode lived up to my hopes and continued the run's solid start. Whether it's generally deemed a good episode or not could hinge on how Before the Flood unfolds, but that's wrong. It should be judged on its own merits.

And after doing so, you can only conclude that Under the Lake is a triumph.

You know what we're getting straight away from an engrossing pre-titles sequence. The writing's on the wall. Ghosts and a base under siege. This sounds like standard fare for Doctor Who, but we've not had these tropes since 2013: Cold War was the previous base-under-siege tale – discounting Mummy on the Orient Express (2014) because it doesn't fit easily into that category – and Hide featured the last use of 'ghosts.'

I love both of these concepts though: the base-under-siege idea works particularly well as it hypes up the drama and the horror so core to Doctor Who. But they only work if you do something new with them.

Fortunately, Whithouse does just this. The function of the underwater base means you get a clever design and smart notions of electromagnetic locks and a night-and-day feature that you know immediately will be exploited. The Faraday Cage, too, is a neat way of keeping the crew alive for the three days that pass between Moran's death and the TARDIS' (unhappy) arrival, and containing the threat later on.

The ghosts, too, are presented in a new fashion - as actual ghosts. The Doctor has never believed in them, so previous explanations typically revolve around time travel or impressions on houses (based on the premise of Nigel Kneale's The Stone Tape). But no, the Doctor has come round to a new way of thinking. These are actual apparitions of the dead.

It becomes even grimmer when the Time Lord realises that they're not natural phenomena at all; instead, someone is hijacking souls...

Things should get even more interesting when we learn how the ghosts came into being and how the Doctor's actions in the past affect Clara and co. in next week's Before the Flood.

But we're getting ahead of ourselves. That's how this episode works: excitement and suspense in equal measure surge the tale on, adding layers of intrigue so you're as interested in these characters and the functions of these ghosts as the Doctor is.

It helps, of course, that you care about these characters. They're all well-realised and fleshed out, despite their back-stories not being explored. You understand their reasoning, their companionship. They're not a slightly-fractured crew like in The Rebel Flesh. The Almost People (2011); they care about each other, right down to Cass (Sophie Stone) telling the Doctor that she won't be held responsible for getting her colleagues killed. More than anything, her closeness to Lunn (Zaqi Ismail) is both endearing and completely understandable. Her concern when he is cornered by a ghost is especially effective, as is Lunn's acceptance that if Cass stays on the base, so must he.

O'Donnell (Morven Christie) is the latest in a long line of fans: she knows and respects the Doctor's work for UNIT, just as Osgood and Malcolm have before. She'll no doubt divide some viewers, but Christie makes sure she's not just a giggling annoyance. She's clever and capable, with a cool persona. Frankly, she's exactly the type of person UNIT would employ!

Bennett (Arsher Ali) is likewise a great character, not as enthusiastic as O'Donnell but nonetheless efficient, dry-witted, and, most prominently, curious. I very much look forward to seeing how he and O'Donnell interact with the Doctor, the past, and the Fisher King next week.

It's a shame we don't get more time with Colin McFarlane's Moran (McFarlane having previously voiced the Heavenly Host in Voyage of the Damned and also starring in shows like Death in Paradise, Dennis the Menace, and The Fast Show), but at least his ghost is wonderfully creepy.

Pritchard, played by Steven Robertson (Luther; Ripper Street) is the most stereotypical, with his underhanded business manner making him an easy candidate to be killed early on. Indeed, his death is especially gruesome, something reminiscent of the classic Robots of Death (1977) cliffhanger in Part One where the Doctor is buried in sand. Similarly, viewing his demise via CCTV (or the 22^nd Century equivalent) reminds this viewer of Rita's death in The God Complex.

In fact, there's quite a lot here that stems from the past. The whole 'catching-ghosts' sequence is similar to the gorgeous scenes in The God Complex where the Doctor tricks the Minotaur into explaining why he's trapped in that spaceship-turned-hotel. Then there's the mole-like Prentis (Paul Kaye) from Tivoli, the same planet as Gibbis.

There are further allusions to The Impossible Planet/ The Satan Pit (2006) and 42 (2007), as well as Sarah Jane, who the Doctor left in Aberdeen instead of Croydon. (Those cue cards for the Doctor are genius, perfect for the Tumblr generation who love to spot easter eggs. The humour in Under the Lake is another marvel: the Doctor especially is hilarious, which is just what we need right now. "Anything else I should know? Someone got a peanut allergy or something?" is a particular favourite line.)

And could Clara's mentioning "that place where the people with the long necks have been celebrating New Year for two centuries" be a hint at the Krillitane, who before taking bat-form apparently looked like us but with really long necks? Furthermore, the mention of synapses in the brain reminds me of The Claws of Axos (1971), although this is likely unintentional.

The weight of these previous adventures doesn't crush Under the Lake. Referencing highly-regarded tales could bury a story under expectations and parallels, but this is an exceptionally clever episode. The blurring between the supernatural and the mechanical (electronics clearly has a lot to do with what's going on) is sublime, as is the tone in general.

And even though the cliffhanger isn't entirely unexpected, it remains an unsettling and enthralling image. More praise must be lavished upon Whithouse and director, Daniel O'Hara for deftly separating the Doctor and Clara, and making an already-cramped space even tighter.

Please excuse such untempered enthusiasm for Under the Lake, but it's already become my favourite Twelfth Doctor story so far. Considering it's got competition from Mummy on the Orient Express and Flatline (2014), that's a great feat.

This is a story that proves you don't have to rewrite history or add a new nuance to the Doctor; you don't have to reintroduce classic monsters, or do something hugely controversial. You don't need these things to make a fantastic Doctor Who story.

You simply need to be fantastic, that's all.

Images: BBC.