The upcoming episode of "Sherlock" season 4 will see Sherlock and John back in action — without Mary — as Toby Jones' villainous Culverton Smith takes center stage.

A trailer for the new episode, titled "The Lying Detective," has been released online. It sees Sherlock (Benedict Cumberbatch) meeting Culverton Smith, who is all smiles as he walks up to the super sleuth. 

"This is the most dangerous — the most despicable human being that I have ever encountered," Sherlock tells an unseen character as scenes featuring John (Martin Freeman), Mrs. Hudson (Una Stubbs), and Molly (Louise Brealey) are flashed across the screen. "This monster must be ended."

It is unknown if Sherlock is talking about Culverton, although his character has been described as "the purest evil" in the past by co-creator Steven Moffat. Culverton is a reality TV star with his own show called "Business Killer." An ad for the show was featured in the premiere episode, "The Six Thatchers." And based on photos that have been released, it seems like he's rather famous for his work.

But what might he really be like for him to be called "the darkest villain?" Fans will have to tune in to Sunday's episode to find out. One thing is for sure, though: Culverton has a great evil laugh.

The previous episode saw one of the most shocking twists in "Sherlock" history. Mary (Amanda Abbington) jumped in front of a bullet to save Sherlock's life — leading to a rift between him and John, who blames the detective for his wife's death. But in the end, Sherlock was sent a CD that shows Mary asking him to "Save John Watson." But what could that possibly mean?

"You can assume we're going to elaborate on that next week," Moffat told Entertainment Weekly in an interview. 

As for Sherlock and John's relationship, Moffat explained things won't just go back to normal immediately. "We take that rift head-on in the remainder of the [season]. We don't ignore it," he said. "We don't have John come back and say, "Well I've thought about it and it's all fine." If anything, the rift gets worse."

"Sherlock" season 4 airs Sundays at 9/8c on PBS/Masterpiece.