LONDON • Sherlock, the world-conquering BBC television adaptation of Arthur Conan Doyle's famous stories, has always been a male duo, an awkward bromance that helped make international stars of Benedict Cumberbatch as the hero and Martin Freeman as Dr John Watson. But the introduction of Mary Morstan, Watson's fianceeto-be, turned the show into something more like a three-hander.
"I like him," Mary announced upon meeting Sherlock Holmes at the start of Season 3, after the hero - who had apparently died in a suicidal plunge at the end of Season 2 - turned up during Watson's proposal to Morstan over dinner. "Short story: alive," he told a stunned and furious Watson, who had been mourning his friend's death for two years.
"Her reaction tells you a lot," said Mark Gatiss, who created the show with Steven Moffat. "She doesn't give Watson the sympathy he wants. It's a clue to what will happen later."
A lot happens later. When Season 4 of Sherlock debuts on Sunday in Britain and the United States, viewers will already know (look away now if you do not) that Mary (Amanda Abbington) is a former major-league assassin, desperate to hide her past from her husband.
But Mary also provides a substantial presence that amplifies the supporting roles of the show's other important female characters: Molly (Louise Brealey), a pathologist in love with Sherlock; and Mrs Hudson (Una Stubbs), his landlady.
"We were very aware from the beginning that women are not very present in the Conan Doyle stories," said Gatiss, who also plays Mycroft, Sherlock's civil servant brother, in the series.
"Mrs Hudson is the most famous woman in the Holmes stories, but even she is a sort of phantom; and Mary Watson really appears in only one story. It was very important to us to have more rounded characters in a 21st-century retelling."
By the end of Season 3, Sherlock is the lone tenant at 221B Baker Street as John and Mary have married, moved to their own house and were expecting a baby. Even before Season 4 begins, fans know that they have had a girl named Rosamund; the producers put a straight-faced birth announcement in The Daily Telegraph, from "your friends, Mrs Hudson, Molly and Sherlock, although he hasn't helped us with this at all as he's always on his phone".
Beefing up the role of Mary (who marries Watson at the end of Conan Doyle's The Sign Of The Four) was a balancing act, Gatiss said. "The thing we desperately wanted to avoid was that Mary was a drag on Watson," he said. "We felt that the woman who marries Watson and becomes part of the Baker Street arrangement couldn't be ordinary."
He added that there had been hints about Mary's nature all along: "You see 'liar' in the text when Sherlock first deduces things about her, she knows skip codes and is far too cool under pressure. Now the chicken has come home to roost."
Abbington said she had been thrilled to discover her character's secret. "I didn't want her to be like so many female roles, propping up the male characters," she said.
"It's always interesting to play flawed, layered characters and she has done horrible things. We worked out early on that Mary and Sherlock have a lot in common. They are all kind of crazy; John is drawn to danger, Mary is a coldblooded killer, Sherlock a sociopath. I wanted to make sure she was as nuts as they are."
For Mrs Hudson, another recurring female character, Gattis and Moffat developed a backstory that gives her an, ahem, interesting past. They also created Molly, who is not part of the Conan Doyle oeuvre. Molly, a shy pathologist in love with Sherlock, first existed, Gatiss said, to demonstrate how chilly and inhuman Sherlock is. But Brealey gave the character a dignity and depth that resonated with the creators. "Louise really did so much with the character that we wanted to see more of her," he said, adding that by Season 3, Molly and Sherlock have developed a real bond that continues into Season 4.
"Molly is a Moffat-Gatiss special," Brealey said. "I tried to make her into a human being rather than a joke, or a caricature. She doesn't just have a crush on Sherlock. She sees something in him, a humanity, that no one else before John was able to see, because he can behave in such a monstrous way." Molly continues to love Sherlock even as she dates other people , and that love, Brealey said, "is strangely redemptive" for both characters.
While Molly is acutely, wincingly aware of every nuance of Sherlock's words and actions, his landlady, Mrs Hudson, is cheerfully impervious to his antisocial behaviour and a source of much situational comedy. "It's a lovely counterbalance to Una's sweetness that she has this interesting past," Gatiss said.
"We'll see more of that in this coming season," he added, mysteriously.
Stubbs said that she had tried to do something different from the austere renditions of Mrs Hudson in other filmed versions of the books.
"I am the mother of three sons, so I thought that would be a good angle to go on," she said. "I once told Benedict that my sons go straight to the fridge and make themselves sandwiches, and he did that in one episode."
Stubbs added that Gatiss and Moffat "have made me more saucy now, and a bit grubby, which I enjoy". She added that she thought Mrs Hudson had seen a lot of misbehaviour.
"So I put up with a lot from Sherlock, although sometimes the curtain comes down and I give him a bollocking," she said, using the British term for a severe reprimand. "I adore him and John, and I like it all nice and cosy."
All the women in the show, Abbington said, are strong, unpredictable, flawed characters. And the women, said Sue Vertue, the producer of the series, see through Sherlock in a way that the male characters usually don't. "The men are all in awe of him, the women admire him, but will tell him off," she added.
However important the women's roles, the show remains centred on Sherlock and Watson.
"I think there is a fine line between updating something and having it true to now, but also keeping it close to the original intention," Freeman said.
"We are rightly concerned with depictions of women, but you can't throw out everything because you judge a different era to be misogynist."
Vertue concurred. "The female characters are there for the right reasons, for stories in which men and women play a part, not to be politically correct," she said. "The women characters are just great characters."
• Sherlock returns to screens on Sunday (timing to be advised) and viewers will be able to catch the new series on BBC Player – simulcast with Britain. It will then be available on BBC First (StarHub Channel 522) in 24 hours.