When Sherlock writer and creator Mark Gatiss addressed the Sherlock Holmes Society of London, in 2006, he spoke of the need to extract the character from the Victorian aspic he had been trapped in and update him for the 21st century.

Three successful and thoroughly modern seasons of Sherlock later, the character is returning whence it came. “There was a hole in the aspic,” Gatiss said recently. “Nature abhors a vacuum.”

Sherlock: The Abominable Bride premieres Friday, January 1, and screens in select theaters the week after. It’s a stand-alone 90-minute episode set in Victorian London that exists in a non-canonical bubble outside of the rest of the series, in the manner of The Simpsons’ Treehouse of Horror.

“The first thing we talked about was having three stories, one of which would be in period,” said Gatiss’s fellow writer and creator Steven Moffat. “We got so excited by putting him back where we found him that we decided this is of course what it should be.”

“In purely practical terms we only had time to make a special and it had to be special,” said Gatiss.

Sherlock Holmes’s own creator, Sir Arthur Conan Doyle, famously became so bored of the character that he tossed him off a waterfall, calling it an act of self-defense. Moffat and Gatiss (the latter plays Sherlock’s brother Mycroft on the show), meanwhile, are dyed-in-the-wool Sherlockians, trading notes on the stories and finishing each other’s verbatim Doctor Watson quotes in conversation. They’re devotees, even, of several generations’ worth of adaptations. “There’s one where he’s a dog,” said Gatiss. “It’s quite faithful! Apart from that one aspect.”

So the duo relished the opportunity to, as Moffat put it, have Benedict Cumberbatch’s Sherlock “just once get the proper gear on.”

“He’s got a pipe,” said Gatiss. “All those things that we’ve always loved which we had to fight against we could just suddenly embrace. I had a long e-mail conversation going, ‘These are the pipes. Here’s a black clay one for disputatious moods.’ We had to sit there and choose magnifying glasses.”

Unlike many Sherlock episodes that adapt existing material from the original stories, The Abominable Bride is a brand-new story, of a kind uniquely suited to the Victorian setting—a ghost story.

“If you want to do a ghost story, you’ve got to go Victorian,” said Moffat. “There’s a lot of fog.”

Gatiss, left, and Moffat. Courtesy of Daniel Boud.

Though displaced, or replaced, in Victorian London, Sherlock retains its contemporary tone and the fizzy repartee between Sherlock and Watson. While the series has a knack for jaw-dropping revelations and cunningly constructed mysteries, Gatiss and Moffat insist that their priority in the show has always been more on their two sleuths than the act of sleuthing itself.

“I think we generate very effectively an atmosphere of puzzles and menace and fast pace and mystery,” said Moffat. “Look at it, though, and it’s mostly about [Sherlock] and his best pal. And there’s enough mystery and suspense and excitement for you to see them more clearly.”

When Gatiss’s season one finale aired, comprising five elegant mini-mysteries, “all anyone wanted to talk about was the last scene in the swimming pool”: the talky showdown between Sherlock and the archvillain Moriarty.

“We often say it’s a show about a detective, not a detective show,” said Gatiss. “That’s true of our version as well as Doyle’s—the mystery generally takes a backseat to atmosphere and character. And we’ve definitely discovered that’s what people love the most.”

Gatiss and Moffat are also two of the forces behind Doctor Who—Moffat the show-runner, Gatiss a regular writer—which means they’re partly responsible for the two most successful BBC shows in the world. Fandom surrounding Doctor Who is a 52-year phenomenon, but Sherlock mania taps into something even larger—and not just the general mania surrounding Cumberbatch .