'Sherlock' season 4 episode 2 spoilers: A cunning adversary arrives amid grief over Mary's death

After one close brush up with Death, "Sherlock" may well find its titular detective, played by Benedict Cumberbatch, back in Samarra yet again when his most chilling nemesis comes to the small screen.

The previous episode of BBC One's modern take on the classic detective series ended on a very devastating note when Mary Watson (Amanda Abbington) took a bullet for Sherlock and her husband John Watson (Martin Freeman) swore his friend off of his life for failing to keep his vow to protect her. However, after Mary's cryptic postmortem message asking Sherlock to "Save John," what would the "functioning sociopath" not do to make sure that he keeps his promise this time around?

In the next episode of "Sherlock" season 4, it's more likely that Sherlock's life, not Watson's, will be in the gravest of dangers when the infamous Culverton Smith (Toby Jones) swings by for a visit. Titled, "The Lying Detective," the episode will be taking its cue from Arthur Conan Doyle's "The Dying Detective," which was one of the final short stories the author wrote featuring his iconic brooding detective.

What exactly is it that makes Culverton "the most dangerous, most despicable human being" that Sherlock will ever encounter, though? The official promo for the next episode shows an agitated Sherlock declaring that this "monster" needs to be ended followed by a montage of the very few people he could truly say he cares about. Are their lives in danger, too?

In an interview with Entertainment Weekly after the shocking season 4 opener, series co-creator Steven Moffat shared that Mary's rash decision to save Sherlock was deeply rooted on her natural protectiveness towards her friends, and that even though "Sherlock" was first and foremost a story about the detective and his friend, Watson, the rift that Mary's death had caused them could only get worse.

"There's a big villain to fight. But front and center are the consequence of Mary's death and Sherlock's culpability," Moffat said.

Where will this devastating loss take Sherlock and Watson? Find out when "Sherlock" season 4 episode 2 airs on Sunday, Jan. 8, at 9 p.m. GMT on BBC One.