After three seasons of modern-day sleuthing, why are Sherlock producers sending their famous detective back in time to Victorian-era England? “Just because we can, really.”

That’s Sherlock co-creator Steven Moffat, talking about the inspiration behind “The Abominable Bride,” the all-new Sherlock special airing this Friday on PBS’s Masterpiece. “Bride” trades the tech gadgets of the modern Sherlock for the horse-drawn carriages of 1890s London — the era of the original Arthur Conan Doyle stories — as Sherlock Holmes (Benedict Cumberbatch) and a mustached Dr. Watson (Martin Freeman) try to crack a mystery wrapped in a good old-fashioned ghost story.

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Moffat and his wife, Sherlock producer Sue Vertue, sat down with Yahoo TV to preview this new Sherlock special for us, explaining why the Victorian setting lends itself so well to a ghost story and promising “porn levels” of inside references for Sherlock fans. Oh, and by the way, Moffat’s also the showrunner of a little show called Doctor Who, so we asked him whether we’ll ever see a Sherlock-Doctor Who crossover coming from his pen.

Steven, you’ve said you wanted to do a Victorian-era Sherlock special simply because you can. How long have you had this idea in the back of your mind?

Steven: It was when we were shooting the little mini prequel, “Many Happy Returns,” for Season 3. [Co-creator] Mark [Gatiss] was directing second unit on it, and I was hanging out with Mark. And that’s when we started talking about it. I can remember the actual day, when we shot the monks with all the candles. We were just like, “Hey, we could just do it.”

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And this special doesn’t tie into the other episodes, continuity-wise, right?

Steven: No, it’s in a bubble on its own. That’s why, when we went and pitched it to Sue, we said it can’t be part of the series. We have to have a special. And if we hadn’t gotten that extra special, we wouldn’t have done it. It had to be on its own. So this is genuinely an extra episode. We had already been commissioned for the three, and then we said, “Let’s go epic and do four."



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Steven Moffat (right) and Mark Gatiss on the set of the Sherlock special (PBS)

Based on the trailer, there are some cheeky, self-aware references in there. So it’s not just a simple mystery. You’re also playing to the fans.

Steven: But that’s sort of unfair to Doyle: He does those jokes! He does the jokes about how Sherlock doesn’t like Dr. Watson’s versions of his adventures, and he negatively reviews them in the actual text. So that self-aware gag, the fact that Sherlock Holmes is a celebrity in his own world who actually reads the stories we’re reading — that comes from the original. You have to do that. There’s a brilliant version of that kind of scene at the beginning of The Private Life of Sherlock Holmes: Billy Wilder’s film, which is lovely. In fact, we just reversed one of the jokes from it, which is, "You saddled me with this improbable costume, which the public now expects me to wear.” We did it about the mustache instead. But you know, the whole show isn’t like that. That’s really near the top of the show, and we’re just sort of easing you into, “This is where we are now.”

Sue: But I think there’s also so many treats there, which…

Steven: Yes. We can’t tell people. But if you’re a Sherlock Holmes fanboy, it’s — [Laughs.] — porn levels of intelligence.

So how much work went into producing this special? Obviously, it’s on another level with the costuming and set design.

Sue: Well, it was. Yes, costume, but also the set. Everything. It was trying to find the equivalent of our wallpaper in Victorian wallpaper. And they had great fun, didn’t they? Kind of getting the alternatives of what we had on the walls. So it still looks very much our show, but in the Victorian era. We did shoot in London, and quite a lot in Bristol. And it’s hard. The clip that you saw, it takes six weeks to close a road down now.