Exclusive: Sherlock Creators Reveal What to Expect From Season Four Finale

Sherlock - The Final Problem
"Sherlock" season finale "The Final Problem" sees Sherlock face his most toughest foe yet... and they're related. Todd Antony/BBC/Hartswood Films

There have been few moments of respite for fans of the hit BBC detective drama Sherlock in its fourth season. First there was the shock death of Mary Watson (Amanda Abbington) in season opener "The Six Thatchers," and in last week's "The Lying Detective" came the explosive revelation that Sherlock (Benedict Cumberbatch) and Mycroft (Gatiss) Holmes have a third sibling, Eurus, played by the relatively unknown Sian Brooke—and she is intent on ruining Sherlock's life.

The reveal sparked a frenzy on social media, but also raised many questions: Where has Eurus been this whole time? Why does she hate Sherlock? And how does Mycroft fit into all of this?

You won't need Sherlock's powers of deduction to figure out the answers. Sunday's season finale is titled "The Final Problem" and show creators Steven Moffat and Mark Gatiss tell Newsweek it will tie up all of the loose ends from the previous four seasons as Holmes and Watson take on their biggest challenge yet.

Newsweek spoke to the Sherlock bosses Thursday ahead of an advance screening of the new episode. Here, they discuss the reveal of a third Holmes, preview the final episode and discuss recent criticism that the show has become too James Bond-like.

Newsweek: How long have you been sitting on this secret sister twist?

Moffat: We were shooting [season three finale] "His Last Vow."

Gatiss: We started thrashing out what we might do next...

Moffat: We didn't mean to. The whole season [came to us], plus [one-off special] "The Abominable Bride." After that, we'd just see each other at some event and we'd go off together and say: "Is it all as good as we think it is?"

Gatiss: The beginning of it was a joke. We had a line in the very first season—it was about Mycroft being cleverer than Sherlock. There was going to be throwaway line where Sherlock says: "It's true, he's cleverer than me, but, mind you, my sister…" and it'd get cut off. We didn't do it—thank God! The idea had been there for a long time but not like this.

Moffat: It's a very simple twist. It's a game of binaries here—brother, brother, brother, sister. The number of people that were surprised by that...there was only one alternative. [Laughs]

The ideal form of the twist is not "I did not see that coming," but "I should have seen that coming." It's easy to surprise an audience but what Agatha Christie proved time and time again, you should surprise them with something they should have worked out for themselves.

Related: 'Sherlock' Star and Creators Talk Huge Season 4 Premiere Twist

What can you say about episode three?

Moffat: Some people are like, "Why didn't Sherlock recognize his sister?" All becomes clear. Nothing is not explained. And it's not a difficult explanation.

It ties up everything we've been doing. It absolutely ties it up. This is Sherlock and John at the top of their game. We've seen them as damaged, wounded beasts, and now we're going to see them doing absolutely what they can do.

Gatiss: It's kind of fighting back. It's a dark season and there's a sense of bring it on . It's relentless, this episode.

Moffat: There is just no occasion to stop. It just keeps blowing up and blowing up.

Does the episode pick up immediately where the last episode left off—Eurus shooting John?

Gatiss: No.

Moffat: Look, what we can tease confidently—because we did it in the trailer —is that we're pretty certain John survived that gunshot. That's just us having fun.

Sian Brooke is incredible as Eurus. How did you discover her?

Moffat: We don't talk enough about casting directors. It's a crucial role. We said to Kate Rhodes-James we need someone who's so talented and so able they should be a star by now but isn't. They couldn't be a familiar face. I'm certain this will make her a huge star.

Gatiss: Her Andrew Scott moment. Or her Benedict Cumberbatch moment.

Last week's episode, "The Lying Detective," was one of my favorite Sherlock episodes ever. Toby Jones was so effectively creepy as villain Culverton Smith. I was following some of the reaction on Twitter when it aired...

Gatiss: Just watch the telly!

Moffat: There isn't a frame we don't worry about. In the time you are glancing down...

Gatiss: You've missed everything!

So, a word to viewers: no Twitter while watching Sherlock?

Gatiss: Of course don't fucking go on Twitter! Watch the telly!

Moffat: And with this episode, there's more in it than you're ever going to get again in 90 minutes. You can't afford to look away. Don't go to the loo—just wet yourself.

Noted. But as I was saying, people on Twitter drew parallels between Culverton Smith and Jimmy Savile. He uses his fame, for example, to gain unprecedented access to hospitals—but little do people realize he's up to no good. Was that intentional in the writing?

Moffat: What we were really going for was how a celebrity is protected by their fame. There are echoes of different celebrities...Savile is the easiest one to go for. In a world where Donald Trump can actually get to the White House, because he's famous even though he's unqualified, talking about the evil that can be hidden by fame seemed like a reasonable thing to do.

Gatiss: And, actually, it's a very relevant, modern thing to talk about. We're always talking about what modern villainy is, but villains don't know they're villains. There are obvious comparisons but we didn't want to make anything too specific. There are so fucking many of these people at the moment who you can imagine, as Toby's character did, holding a press conference and saying: "I never said that." And everyone goes: "Yes, you did." But they just get away with it.

Toby Jones in Sherlock
Toby Jones as the creepy Culverton Smith in "Sherlock." Robert Viglasky/BBC/Hartswood Films

Mark, you wrote a poem recently in response to a Guardian review of "The Six Thatchers." The critic said the show had become too much like Bond...

Gatiss: It was either that or a dirty limerick. I don't read reviews. This one was brought to my attention. It made me cross because I knew it's not true and the guy who wrote it knows it's not true. Sherlock Holmes has always been a man of action. [Arthur] Conan Doyle responded in verse a hundred years ago so I thought I'd have a go. What's amazing is that it's had more reach than any interview we've ever done! I should write more poems. [Laughs]

It was ingenious to send it into the letters' section of the Guardian...

Moffat: Doyle did this. There was a critic at the time who used to write in verse and he wrote something silly about Sherlock Holmes. Doyle answered in much, much better verse about not acquitting him with his creation.

Steven, we know what Mark thinks of the Bond criticism. How did you react to it?

Moffat: There was a three-minute punch-up in the middle of the episode. If you see a Bond film where the action is restricted to a three-minute punch-up, let me know and I'll have it deleted. This episode on the other hand… wait and see.

Season four of Sherlock concludes with "The Final Problem" Sunday on BBC One in the U.K. and PBS in the U.S.