We always knew it would come down to this.

From the moment it was first announced that John Simm would return to Doctor Who as the maniacal Master — a role he hasn't played since the David Tennant era — we knew that the final climax of the season would almost certainly involve the Master, Missy (Michelle Gomez) — the current incarnation of Simm's character — and the Doctor (Peter Capaldi) coming face to face in an epic showdown. And although it obviously would have been much better and more shocking if Simm's return hadn't been announced in advance to feed the publicity machine , the first half of the two-part finale still delivered on the drama and angst.

In fact, Saturday's "World Enough and Time" turned out to be one of the best and darkest hours Doctor Who has seen in years. Written by exiting showrunner Steven Moffat, it was a major improvement over the middle portion of the season, which sagged in comparison to the reenergized episodes that opened Capaldi's final turn in the TARDIS. The writing, the pacing, the performances and the direction of Rachel Talalay all combined to create an exceptionally emotional and gripping hour of television. Perhaps we shouldn't be surprised by any of this though: the episode's cold open featured the Doctor, hair longer and wilder, at the start of a regeneration. It was a warning that said the end was no longer just near, it's finally here, and this knowledge created a sense of tension that would extend to everything that came after.

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However, the scenery-chewing Master was not the only familiar face returning to cause the Doctor grief this week. The episode also featured the return of the Mondasian Cybermen — one of Capaldi's favorite villains and the original version of one of the series' most popular monsters. First introduced in the 1966 episode "The Tenth Planet" and responsible for the death of the first incarnation of the Doctor, the Mondasian Cybermen made for extremely creepy villains here. Their cloth faces, human-like hands and simple design could have been — and probably should have been — laughable in 2017, but the simplicity of it all actually added to the horror of the situation.

Michelle Gomez, Peter Capaldi and John Simm, <em>Doctor Who</em>Michelle Gomez, Peter Capaldi and John Simm, Doctor Who

Because it wasn't just that we were witnessing the genesis of the Mondasian Cybermen that was frightening, it was also the reveal that Bill (Pearl Mackie) had been converted into one after being shot through the chest. This is the kind of bold storytelling that Doctor Who has been missing. It's the kind of dark turn we thought we were going to see during the three-episode long Monks arc but which never came. It is the kind of risk the series needs to be taking more often if it wants to break free of the rut it keeps finding itself in, year after year.

No, that is not one last parting shot at Moffat, who sometimes struggled with overly clever longterm arcs as showrunner (for what it's worth, he is also responsible for some of the show's most unforgettable adventures since its return in 2005), it is merely a statement about the series' repetitive nature. There are only so many times the Doctor can believably save everyone before our own ability to believe in the unbelievable is worn thin. That there are going to be casualties along the way is far more realistic for the very reasons the Doctor told Bill he couldn't promise her she wouldn't be killed in Missy's test run: humans are mortal. We break quite easily and don't come with spare parts. Our short lifespans by contrast are what makes the Doctor's own longevity unique — and rather sad. But what happened to Bill is unbearable, and the consequences of what happened — and the Master's role in what happened — will likely be felt for a while.

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Because this isn't just about Bill. This is about the Master and Missy and the Doctor's desire to see his oldest friend — the only person even remotely like the Doctor — become a good person. Before Simm's version pulled off his disguise (the man loves himself a disguise) Missy was showing some signs of, if not changing, at least being capable of change. She did, of course, poke fun at playing the role of the hero — and Gomez's comedic timing was possibly the best it has ever been — but she didn't appear to be actively sabotaging the Doctor ... until she was.

Michelle Gomez, Pearl Mackie and John Simm, <em>Doctor Who</em>Michelle Gomez, Pearl Mackie and John Simm, Doctor Who

Missy appeared to have, perhaps understandably, joined forces with her goateed former self by the end of the hour, which puts her winking about receiving a distress call at the beginning of the hour in a new light. And with the Master returning, "World Enough and Time" — which gets its name from the metaphysical poem "To His Coy Mistress" by Andrew Marvell — played with time in a way the series hasn't really attempted lately. The two versions of the Master complemented the timey wimey nature of the spaceship, which was affected by a blackhole so that time moved more slowly at one end of the 400-mile long ship than it did at the other. While the idea that the Master was playing a long con isn't new, this time the Doctor couldn't save his companion even though she did as he'd asked and waited for him. And that's no doubt going to hit him hard.

As we head into the final hour of the season, the rumors that Mackie won't return as Bill next season and will depart the series alongside Moffat, Capaldi and Gomez, thus giving incoming showrunner Chris Chibnall a blank slate to work with, appear more and more likely. It's unfortunate that we'll have had only one season with the immensely likable Bill Potts, but if this is what it takes to bring dramatic stakes back to Doctor Who, it's also worth it.

Doctor Who airs Saturdays at 9/8c on BBC America.