Yes, there are spoilers. Lots of them.

I have no idea how someone new to the program would react to this one, but I thoroughly enjoyed it. Nods to everything from the Second Doctor stories Tomb of the Cybermen and The Invasion (which apparently gets an even more tangible link next week, when Kate Lethbridge-Stewart throws the head of one of the Invasion-era Cybermen at the feet of the new ones), to the setup of a standard Third Doctor vs the Master allied with alien invaders of Earth story, to the Cloister Bell as the first TARDIS key hits the lava (first heard in the Fourth Doctor’s Logopolis), to the introduction of Kamelion in the Fifth Doctor’s King’s Demons (though that involved an android briefly impersonating the Master, and not the other way around), to the young Chang who was suborned by the Master in the Eighth Doctor’s TV movie (“Doctor Chang!”), possibly to The Eleventh Hour (Missy walking through the 3W hologram as the Doctor walked through the Atraxi one), to Day of the Doctor (The Doctor: “You’re very… uh… realistic.” Clara: “Tongues?” The Doctor: “Shut up.” / Eleven: “venom sacs in the tongue?” Ten: “yeah, I’m getting the point, thank you.”), and to Time of the Doctor with Clara’s disagreement with something in another universe about what the important question is, not to mention multiple callbacks to episodes from this season. Then there are musical motifs: the Doctor saying “I feel like I’m missing… something… obvious”, cue ominous music, but not just any ominous music, that’s the Cyberman theme from series two. And a cheeky nod to Capaldi’s best-known previous role as the very sweary Malcolm Tucker (Doctor Chang, examining the psychic paper: “Why is there all this swearing?” / The Doctor, taking it back: “Oh. I’ve got a lot of… internalized anger.”)

And, with a female Master onscreen, a future female Doctor becomes canonical (the possibility was already allowed for by a line in the Neil Gaiman story from series six), answering a question which has been in the air ever since Tom Baker was asked thirty-five years ago if he thought his successor could be a woman, to which he mischievously replied “Oh, what a good idea!”

Back to Dark Water. First off, it’s a Moffat two-parter, so it is very unlikely that it will stay in the Third Doctor and UNIT vs Master allied with aliens invading the earth, cue likely falling-out between aliens and Mistress and possible alliance between Mistress and Doctor to contain the threat. (Hmm. Aside from UNIT, how very End of Time.) As Amy said towards the beginning of The Big Bang, which turned the already-complicated Pandorica Opens on its head, it will probably get more complicated than that. No idea how, but looking forward to finding out.

One more prediction: in the last moments of this episode, Danny sees the reflection of the civilian boy he killed as he is considering deleting his humanity (and, unknown to him, being re-downloaded into a Cyberman). Given his strong sense of responsibility for the children in In the Forest of the Night, I expect this will cause him to decide not to delete his humanity, but to work with the boy, to find out where they really are and what they can do about it. Though if Danny’s dead body already has a Cyberman exoskeleton around it, and besides which is dead, and the Doctor has stated that they can’t use the TARDIS to fix that (and yes, I know, the Doctor lies, but…), I’m not sure what can be done. Though there’s also the question of what happens to Orson Pink in that case.

The episode itself? Felt really really good. No, there’s no scientific reason why Cybermen would still need organic components (the skeletons) or downloaded organic minds (with icky emotions removed), but they’re here as pure horror, not hard science fiction. Or we’re back to the idea that they aren’t and don’t see themselves as robots, but as upgrades of living things. And apparently on a voluntary basis this time. “We are the Cybermen and we are here to help”?? Although the way Danny is emotionally sucker-punched by being exposed to the parts of his life most likely to cause him to willingly agree to delete his emotions suggests either that the not-emotionless Missy or Seb gave them some pointers on how to improve their “voluntary” success rate, or they’ve been running A/B tests for so long on how to get the most voluntary conversions (since they really believe it’s for the good of the recipients) that they’ve come up with a strategy so effective that it looks like emotional insight.

Clara’s post-its as she prepared to answer Danny’s question from last week and tell him the truth, and then started with the most important thing, which was nothing to do with the adventures? And then killing Danny in the cold open? And Clara’s response? “It wasn’t terrible: it was boring. … He was alive, and then he was dead, and it was nothing.” / “I don’t deserve anything. Nobody deserves… anything.” [“Od’s bodykins, man, much better: use every man / after his desert, and who should ’scape whipping?” – Hamlet II.ii] “But I am owed… better.” Still bitterness from Kill the Moon in that last phrase? Entirely possibly. And then the seven keys to doomsday (sorry, to the TARDIS, though the number of keys is probably a reference to Seven Keys to Doomsday, a Doctor Who stage play in 1974, re-released as a Big Finish audio in 2008), one of them hidden in Audrey Niffenegger’s The Time Traveller’s Wife (which could be subtitled the story of the Doctor and River Song), and then the volcano scene which acknowledges that sometimes time can be rewritten and sometimes it can’t, and it takes a Time Head to tell which is which, and even if it can it takes precision and great care. And the whole scene calling back to Journey Blue in Into the Dalek trying to threaten the Doctor at gunpoint to taking her back to her ship, and him outfacing her until she says please. Or the Deep Breath interrogation scene with its discussion of options and fallbacks. “Paradox loop”, as the Doctor desperately tries to get Clara to understand, knowing that River would understand instinctively, and perhaps forgetting that in The Wedding of River Song River did know but since it was someone she loved dying she decided she didn’t give a damn about the rules either. And then “Do as you’re told”, and we’re back to the fraught conversation near the end of Listen, except this time it ends differently. And then she throws the last key and… “I’m sorry, but I’d do it again.”… The acting from both Coleman and Capaldi in this scene is superb. (And never mind that the Doctor can open the TARDIS by snapping his fingers, and for that matter so can Clara. That’s not the emotional point of this scene.) And it just keeps going. “I love him.” “Yeah, you’re quite the mess of chemicals, aren’t you.” Capaldi’s slight frown after Coleman says “Absolutely fair enough”, before explaining what he actually meant, leading up to the transcendently wonderful “Do you think I care for you so little that betraying me would make a difference?” And then the Doctor’s wholly alien response to Clara’s entirely human reaction? I mean, yes, probably also true that “We’ve got work to do” is a reference to the Seventh Doctor story Survival, but that scarcely seems to matter. These may be my favourite scenes from the whole fifty-one-year history.

And then we’re back to Danny in the Nethersphere, and Chris Addison being delightfully camp as the Nethersphere administrator Seb. “There are some forms to fill in. It might help you relax. Well, it won’t, but we do need them filled in.”

And Twelve continues his strange mix of not understanding humans at all, and suddenly seeming to understand them better than they do themselves, berating Clara for her appearance and attitude until she bursts out “Are you forgetting why we’re here?” and he retorts “We’re here to get your boyfriend back from the dead. So buck up, and give me some attitude.”

And then Michelle Gomez as Missy turns up, and that’s a totally brilliant scene too. “My heart is maintained by the Doctor”, indeed. “You can probably take your hand down now, Doctor.” And then we get the Doctor being as protective as a can-opener [Gaudy Night]: “Are you okay?” (Clara holds out her hand) “No.” (The Doctor takes her hand) “Good. There would be something very wrong if you were.” (And the skulls turn their heads to watch them as they go, recalling the crypt in The Wedding of River Song, but working so well it doesn’t really matter.)

And then Danny gets the call from someone on the other side called Clara Oswald. (And at the same time Doctor Chang says “I promise you, this is not the con,” which leads to the question what would he admit was the con, which we’ll presumably get to next episode.) In Time of the Doctor, all of Gallifrey was behind the crack in the skin of the universe, and was asking the question “Doctor Who?”, a question which only the Doctor knew the answer to, and so if he gave that answer, they would know it was safe for them to come back. Near the end of that episode, Clara spoke through the crack and told them they were asking the wrong question, that his name was the Doctor, and that if they loved him, and they should, they would help him. Back to Dark Water, Clara is talking with Danny, and after the Doctor primes her again to be skeptical, wants him to answer questions that only Danny would know the answer to, so she can come and get him. He, since he knows that he’s dead and she isn’t and shouldn’t be, refuses to give her the answer she is asking for, saying only that he loves her. Which is so sublimely in character for Danny (and such a callback to the sequence in Time of the Doctor and to two earlier conversations this episode) that though Clara appears not to realize it at first and breaks off the call, the penny may have dropped as she backed her chair away, and certainly will sometime before the end of next episode.

But Clara is distracted by the revealed Cyberman of Dr Skarosa, and let’s pause there a moment, because Missy making her pet scientist stooge take a name clearly based on the Dalek homeworld of Skaro feels like such an obvious clue to the Doctor (if he were paying attention) that it reawakens the long fan-held suspicion that all of the Master’s plans are so ridiculously flawed that they have to be more about attracting the Doctor’s attention than actually taking over the world. Which makes sense of Missy’s delivery of the “my boyfriend… because he loves me so” lines from Deep Breath, which seemed a bit off if it were something she actually believed, but if it’s something that she has wished so long were true but knows on some level is not, her delivery is brilliant and makes perfect sense.

Then we’re back outside to the reveal of who Missy actually is (the looks on both of their faces worth pages of dialogue) and the Cybermen walking down the steps of St Paul’s, and through the eye of one of the Cybermen (top-notch direction from Rachel Talalay throughout, and also a likely shout-out to The Time of Angels and the eyes being not the windows of the soul but the doors) we’re back to the Nethersphere and Danny hesitating over the delete button and turning the reflection of the iPad to reveal that the boy he shot is standing behind him, and the boy starts to speak… cue closing credits.

Damn near perfect. There’s three words for it.