The wait is finally over. Sherlock returns on February 15 and the fourth series is the darkest to date, according to creators Steven Moffat and Mark Gatiss. 

Trying to prise details about a new series of Sherlock from creators Steven Moffat and Mark Gatiss is like playing amateur sleuth with a minimum of clues. But preserving an element of mystery has always been a priority, and synonymous with the nature of the show.

“The roads to wrath have demons beneath and Sherlock’s have been waiting for a very long time,” whispers Gatiss cryptically, before admitting this is actually a quote from series four.

“There are consequences to the kind of madcap, in-the-moment fun lives that Sherlock and John and Mary lead,” explains Moffat. “There are things that have happened, there are enemies that they have made, there is damage that has been done and some of that is coming back to visit them. There will be surprises, and when some of those surprises happen you’ll think, ‘ah, I should have seen that coming.’”

One of those surprises is a new arrival in the lives of Watson (Martin Freeman) and Mary (Amanda Abbington) in the form of a baby daughter. But how will parenthood affect their relationship with the mercurial Sherlock?

“I think Sherlock feels very protective towards them as a family, but he’s not a natural or a figure of authority when it comes to a newborn,” says Benedict Cumberbatch. “He’s seemingly indifferent, which is comic at times but it’s all underpinned with a deep love, and he’s a guardian angel really.”

To a greater or lesser extent, the series has always taken its cues from the stories of Sir Arthur Conan Doyle, modernising them and adding quirky riffs on some of the classic titles (The Hounds of Baskerville, The Sign of Three, A Scandal in Belgravia).

Gatiss reveals that the 1904 adventure, The Six Napoleons, inspires the events of the series four opener. “It’s called The Six Thatchers now,” he adds, “but that book was the main inspiration, although it’s the bare bones of the story.”

He also hints that this season will venture into some dark and horrific places, “particularly episode three – it’s like a Universal horror film with Basil Rathbone.”

“It’s a cliché to say it’s the darkest season, but it is,” he continues. “It’s the darkest but also the most meaty, with the most proper dramatic incident we could possibly throw at them all.”

Despite all the darkness, Amanda Abbington was thrilled to be back on the set. “I love Sherlock and I love being a part of it, and this series is particularly exciting and dramatic. It was great to be back with everybody, with loads of old faces and some new ones as well.”

Moffat and Gatiss will neither confirm nor deny that one of these old faces is Moriarty’s, but in a choice piece of casting, veteran actor Toby Jones joins series four as diabolical villain Culverton Smith. Abbington notes that Jones’ performance is “truly terrifying” and he remained in character between takes. “He frightened the bejesus out of us on set,” she admits.

Series four is the first full season of Sherlock in three years (although last year’s special, The Abominable Bride, reduced the wait), but will it be the last? With both Freeman and Cumberbatch now a part of the Marvel Cinematic Universe, it must be harder to get them both together?

“It’s very difficult,” says Gatiss. “One of the great things about the job is it’s only every few years, so there’s room to do Doctor Strange and whatever in between and then come back to it. Both Martin and Benedict are very grateful for what the show has done for them, and hopefully we’ll carry on as long as we can.”

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