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, Dr. Watson’s fiancée-to-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 Dr. Watson’s proposal to Ms. 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 Jan. 1 in both Britain and the United States (on Masterpiece on PBS), viewers will already know (look away now if you don’t!) 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.