In Season 4 of Sherlock, Benedict Cumberbatch’s dashing detective comes up against a new, never-before-encountered adversary. After having outwitted several criminal masterminds, infiltrated a Chinese smuggling ring, uncovered a military conspiracy, foiled a terrorist plot to blow up the Houses of Parliament, and faked his own death, it may be his greatest challenge yet—a baby.

“It’s not Two Men, One Woman and a Baby,” series co-creator Mark Gatiss says of the arrival of John and Mary Watson’s child. “But we do have fun with it. The notion of Sherlock having to be around a baby is just funny and intriguing. Because he would approach it like a case. He would probably read up on it and think, I can do this. But babies aren’t logical.”

Gatiss adds: “But it’s not suddenly some sort of rom com.” He pauses for a moment. “That’s Episode 2.”

That’s a joke—although the creators of Sherlock aren’t averse to barefaced deception. Speaking to Vanity Fair last year in the lead-up to the Sherlock Christmas special, “The Abominable Bride,” Gatiss and fellow show-runner Steven Moffat encouraged the assumption that the episode, set in 19th-century London, was a one-off, stand-alone affair, separate from the series proper.

The truth was slightly more complicated. Much of “The Abominable Bride” turned out to be taking place in Sherlock’s drug-induced imagination, and the “stand-alone special” in fact stood very much alongside the “regular” episodes. It was an elaborate hoax culminating in a meta-twist worthy of a show about fiction’s most celebrated sleuth.

“We lied to you,” Gatiss admits, more gleeful than apologetic.

It helps that Gatiss has the sincere-seeming “soft, precise fashion of speech” originally ascribed to the criminal mastermind Moriarty. A certain amount of cunning comes with the territory: perhaps inevitably for a show awhirl with ingenious riddles and mysteries, Sherlock has inspired a following of would-be detectives obsessed with picking up clues and unraveling the show’s secrets.

Gatiss, who also stars on the show as Sherlock’s brainier brother Mycroft, is determined not only to outwit the Sherlock sleuths, but to also confound the expectations of those who know the old stories. “It’s such a spoilery age,” he says. “People demand things all the time. But, genuinely, if you gave it to them, they’d be disappointed. It’s so wonderful if you can maintain it. It’s marvelous to keep your secrets.”

Among the mysteries fans are currently salivating over is the hinted existence of a third Holmes sibling, possibly named Sherrinford (Gatiss: “Well. We’ll see. The clues are there . . .”), and the nature of the posthumous return of Sherlock’s arch-nemesis. (“Moriarty is dead,” Gatiss insists, adding, “More importantly, Sherlock knows exactly what he’s going to do next.”)

Spoilers aside, there’ll be the usual astonishments in store in Sherlock Season 4, whose three 90-minute episodes—“The Six Thatchers,” “The Lying Detective,” and “The Final Problem”—will air weekly in the U.S. from January 1 on Masterpiece on PBS. The last episode lifts its title from the original story in which the Sherlock Holmes character was killed off, plummeting off the Reichenbach Falls; the earliest promotional image for the season was of a violin with one of its strings broken.

What’s certain is that the series will feature a new villain in Culverton Smith (played by Toby Jones), whom Gatiss has described as “purest evil.”

“The danger with anyone other than Moriarty is you run the risk of them appearing as a diluted version,” he says. “Thus our other villains are very different: Magnussen was a businessman in the Murdoch vein—not evil as far he’s concerned. Just totally amoral. Culverton Smith is different again—you’ll have to wait and see!—but very much a man of these strange, rootless, dark times. What can you not do if you have power?”