SPOILER ALERT: This article contains details of tonight’s Season 4 finale of Sherlock.

“The Final Problem,” the last episode of Season 4 of the hit BBC One/Masterpiece drama, Sherlock, aired tonight in the UK and U.S., completing the long-awaited three-part return of Benedict Cumberbatch as the high-functioning sociopath sleuth, and Martin Freeman as sidekick Dr John Watson. After a season that began with the shocking death of another main character, and a second episode that boasted one of the creepiest villains ever in Toby Jones’ Culverton Smith, tonight’s “Final Problem” pulled out all the stops.

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BBC
Among some of the elements featured in the thrill-ride of an episode: a crashing plane, an isolated island prison, elaborate disguises, surprising weaponry, mind-bending puzzles, gruesome allusions, sincere emotion, violin interludes and, a spine-chilling meet-up between the third Holmes child, Eurus, and Sherlock’s arch-nemesis, Jim Moriarty.

Yes, he’s back — kind of. Teased essentially ever since his demise in the Season 2 finale, “The Reichenbach Fall,” Andrew Scott as Moriarty makes what can only be called a rock-star’s return in “The Final Problem.” More on that below.

The real threat here is Sherlock and Mycroft’s sister Eurus (Sian Brooke) who first appeared in this season’s debut, “The Six Thatchers” and then again in last week’s “The Lying Detective” — albeit as different characters. Much is revealed about her backstory and the Holmes’ childhood — including the mystery surrounding Redbeard who we’ve long believed was Sherlock’s beloved dog.

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REX/Shutterstock
The BBC hosted a screening of the episode for fans and the press last Thursday in London, followed by a Q&A with co-creators Steven Moffat and Mark Gatiss (who also plays Mycroft), producer Sue Vertue, director Benjamin Caron, and co-stars Rupert Graves, Scott and Brooke. The gang discussed the Season 4 finale and whether it truly spells the end of Sherlock’s run, as well as diving into some of the overall themes and twists throughout the series.

In “The Final Problem,” Eurus puts Sherlock, Watson and Mycroft through a maze of tests (some quite deadly) with puzzles around each corner of the high-security island prison, Sherrinford, where she has been socked away for decades. As the episode progresses, and in one particularly charged scene involving Molly Hooper (Louise Brealey), Sherlock continues to get in touch with his emotions and comes to understand, as Moffat said, “that he’s stronger and smarter than Mycroft. Not because he’s actually smarter — he’s less smart — but because his emotions, his connections to other human beings, the wisdom that he has gained from the connections he has made in the world, make him stronger.”

Moffat added that the lessons Sherlock has learned over this season (in which he is “slightly less of a dick” as was noted at the screening for the first episode) are related to the sister he had forgotten.

“The extreme of Eurus, who has no connection to anything, is just pure brain. Not understanding anything about what it is to be human makes (Sherlock) realize that (the fact that he has) worked towards everything that he has tried to step away from and deny, is what makes him the stronger one. He isn’t as smart as Eurus or Mycroft, he’s just always going to win against them because he is better and stronger and that is him becoming the Sherlock Holmes of Basil Rathbone and Jeremy Brett… The wise old man of Baker Street we’re used to who’s still terrifying and cold but has a heart that we never doubt.” The crowd at the BFI heartily applauded that sentiment on Thursday.

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BBC
Promos have shown Sherlock uttering what might seem an unthinkable phrase coming from his lips: “I love you.” The words are said during a tense stand-off created by sister Eurus in “The Final Problem,” and are directed at pathologist Molly. But the scene almost didn’t happen. Said Moffat, “There was a completely different scene there, and two people liked it. That was me and Mark. Everyone else hated it. On the very last day we spent writing this episode, we started looking at it and said, ‘You know what? Everybody thinks this is shit. Let’s just go to our office and think of something else.’ So we did and it became ‘That’s the best scene in the episode.’ That’s the value of getting really negative feedback.”

Added Gatis, “The emotional turmoil is much stronger especially in the scene with Molly… There’s a ticking clock and she has to say something she can’t bring herself to say and he can’t bring himself to say. It’s really very powerful.”

As for Scott, whose re-entrance had the BFI audience hooting and hollering last week, he said he’s been asked the question of whether he is still part of the Sherlock universe, “every day” since “The Reichenbach Fall” aired.

The Moriarty scenes actually date from five years prior in the narrative, so no, he’s not still alive. Moffat exclaimed, “Can we point something out? In total fairness, we didn’t lie, he’s really dead, gone.”

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BBC
Moriarty and Eurus have a particularly chilling and almost erotic encounter between psychopaths. Gatiss called it “wonderful” saying, “evil met evil across a glass divide and morphed into one another.” The discussion between the two characters is never shown, but its result becomes very clear later in the episode.

Brooke said, “It was much more effective to see that scene as a meeting of psychotic minds. Words are unnecessary.” Scott echoed, “It was actually very good that we don’t see the conversation. The idea of just seeing them physically size each other up is a little more interesting.”

Discussing how Eurus ended up being a sister rather than the long-suspected third Holmes boy, Moffat said that when working on Season 3’s closer, “His Last Vow,” the co-creators had “an accidental chat in the accountants office — which is the top layer of a double-decker bus — and the whole story from ‘The Abominable Bride’ is what we talked about that day and the madness that we thought we could never sustain of hinting that Sherlock’s got a brother and then pulling, frankly in the circumstances the only twist you can, which is that it’s actually Sherlock’s sister.”

With Redbeard a key element to “The Final Problem,” Gatiss explained another switch — shifting the character from canine to human. “I remember being in (exec producer) Beryl (Vertue)’s flat talking about something else, and I thought ‘What if Redbeard’s not a dog?’ And then everything kind of clicked from there. Redbeard began as a joke and we sort of took out the cruelty of Mycroft tormenting (Sherlock) and it became this thing which sometimes you plant a seed and it grows sort of monstrously.” Eurus’ character and her involvement in Redbeard’s demise, “was all about trying to go somewhere new.”

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Hartswood Films/Todd Antony
Certainly not new is the question of whether “The Final Problem” really equates to the final time we’ll see Cumberbatch and Freeman as the Baker Street Duo. Neither Moffat nor Gatiss committed to the thought, with each noting that there continues to be a desire to continue. Said Moffat at one point, “If this is the last time — and I’m not planning on it to be, but it might be — it is possible that we could end it” with this episode. “We couldn’t have ended it on any of the previous series because there was what have been great cliffhangers.” Added Gatiss, “It may be the final problem, you never know.”

Explaining how the series has evolved, Gatiss said, “I think what has actually happened is we have now done the story of how the Sherlock Holmes and Doctor Watson that you’ve always known became those men. It’s actually a backstory and we never intended it to be, but the reason to leave it at that place is that actually if we do come back — and we’d love to come back — then we could have it absolutely start with a knock on the door and Sherlock saying, ‘Do you want to come out and play?’”

Moffat also heaped praise on wife and producer Sue Vertue. “If we never come back and we never do another panel on the show, which is improbable, there is one thing I would like to say and that is, and it’s not said enough and it sort of goes by unnoticed, is that the boss of Sherlock, the No. 1 person sitting in front of you right now is not me or Mark, it’s the person to whom I’m married. It is absolutely true she is the person who runs all of Sherlock. She’s the person who either kicks us up the arse or restrains us depending on whether we’re being insane or lethargic. It has been her project from the very beginning. She’s the general, the chief executive officer, the absolute ultimate boss of this show. If you love Sherlock, the person you have to thank the most isn’t Benedict, Martin or Mark or me. It’s my wife Sue Vertue.” Cue more applause.

One final note: Keep an eye on the dead Viking in the 360° scene at the end of “The Final Problem.” He was described by director Caron as “a good friend of Martin.”

After the episode aired in the UK tonight, BBC One posted the following to Twitter: