This preview of Doctor Who: World Enough and Time contains references to elements of the episode that have already been officially revealed by the BBC but no major spoilers beyond that.

After the endearing freshness of series opener The Pilot and the lovely Frost Fair setting of episode three Thin Ice, I've found this series of Doctor Who slightly underwhelming. But as we enter the home straight for Peter Capaldi's Twelfth Doctor and Steven Moffat's tenure as showrunner, it feels as if things are stepping up a gear (to slightly mix a metaphor). This weekend's offering, penned by Moffat, is not only one of the best episodes of this run but one of the best since the show returned in 2005 – and certainly one of the darkest. 

World Enough and Time is officially the first in a two-part series finale but, given that it opens with a dramatic foreshadowing of Capaldi's departure in the Christmas special, it may effectively be the start of a three-part farewell.

The one thing I can tell you about that scene without spoiling it is that Capaldi's hair is wilder and more majestic than we've ever seen it before.

After that pre-credit cliffhanger, we're back in the present, whenever that is, sweeping along the flank of a giant spaceship so gorgeously realised that it would be at home in a blockbuster movie. On board, Missy is sallying forth on her first training exercise as "awesome hero" "Doctor Who", with "expendables" Bill and Nardole in tow and the apparently repentant Time Lady on excellent comic form.

The Doctor is monitoring proceedings from the Tardis and – just to underline the distinct lack of peril that's about to evaporate in an instant – he’s stuffing his mouth with crisps as he issues instructions.

But then – and before I say this, bear in mind again that this is a largely spoiler-free preview – a thing happens. It’s quite a significant thing, that might make some viewers feel emotions. And it results in Bill being separated from the Doctor. 

But it's where she finds herself next that things start to get really dark. The setting is the fantastically eerie clinic we've already glimpsed in trailers and promo pictures and Bill hears a sound there that leads her to a horrible realisation, expressed in the most macabre way possible. It puts me in mind a little of one of the ideas that so disturbed viewers in the opener to another recent Doctor Who series finale, Dark Water. Forget the children, I wonder how many adults will have nightmares after watching this.

There are of course a number of other similarities between series eight's Dark Water and World Enough and Time. Both are the openers to two-part finales, both feature Missy and both involve an army of Cybermen. But here it's not just the unthreatening action-figure versions but the original Mondasian Cybermen encountered by the First Doctor in 1966 story The Tenth Planet – and god are they creepy. Those ghostly masks and human looking hands always made them the most disturbing Cybermen but their backstory, as revealed here, really turns the horror up to 11.

Bill at least finds an ally among the gloomy corridors and sickly patients of her new home, a character who manages to impart warmth and plenty of black humour and who you feel may become more significant as the story proceeds.

But wait, I'm forgetting something... oh yes, the Master! Anyone who actually watches Doctor Who will know by now that John Simm's maniacal incarnation of the Doctor's Time Lord nemesis – Missy's former self – returns in this episode with a goatee beard and a diabolical plan. Exactly how he arrives you'll have to wait to see but despite the fact that it's been heavily trailed it may still take some people by surprise.

We end on an appalling final revelation asking ourselves some big questions about more than one key character. I've just watched that scene again and I'm shivering on what is a hot day in London – partly with horror and partly wth anticipation...

Doctor Who: World Enough and Time is on BBC1 on Saturday 24th June at 6:45pm