While there is no definite date yet as to when "Sherlock" returns for its fourth season, one thing is certain: It returns with a new batch of mysteries.

The BBC network has yet to make an official announcement on the air date for the new run. However, director Benjamin Caron confirmed recently that filming for the first three episodes has already been completed.

According to show creator Steven Moffat, the new episodes can be described as an "emotional upheaval."

"We're certainly going to put them through the mill," he told Entertainment Weekly.

"It's going to be more of an emotional upheaval. Hopefully enjoyable and fun, all the things Sherlock must always be. It will be tough at times. Maybe that's the word? A tougher series," he added.

In the season 3 finale, Moriarty, who was supposed to be dead, appeared on every screen in the country, uttering the words, "Did you miss me?"

Moffatt teased that there will be no last-minute whim in the return of Moriarty. "We've had what we're going to do with Moriarty in place from before the second season," he told Vulture.

He also revealed that the premiere episode of the upcoming season will see John and Mary become parents. It may be recalled that in the previous season, it was revealed that Mrs. Watson was a secret assassin.

Meanwhile, Toby Jones will play Culverton Smith in the second episode. In "The Dying Detective," he took advantage of his knowledge in deadly tropical diseases to kill Sherlock.