Showrunners Mark Gatiss and Steven Moffat did a splendid job in concealing the identity of Sherlock (Benedict Cumberbatch) and Mycroft's (Gatiss) infamous sister Eurus (Sian Brooke). As the show approaches its season 4 finale, titled "Final Problem," the Holmes brothers and Dr. Watson (Martin Freeman) are expected to go through one final challenge that will make them realize they are no longer playing a complex game.
"Every choice you've ever made, every path you've ever take — the man you are today is your memory of Eurus," Mycroft tells Sherlock in the recently released trailer of the finale.
The same trailer also shows Watson addressing Sherlock, saying, "Look, I know this is difficult, but you've got to keep it together."
In a previous interview with Entertainment Weekly, Moffat explained how they intend to challenge Cumberbatch and Freeman in season 4. "We're certainly going to put them through the mill," Moffat revealed. "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."
The most shocking revelation in all of the "Sherlock" seasons would have to be that Sherlock and Mycroft actually have a sister who has been disguising herself as three different persons. She appeared as Dr. Watson's therapist, Culverton Smith's daughter in episode 2, and the woman in the bus whom Dr. Watson flirts with. She revealed herself as Sherlock's sister to Dr. Watson then proceeds to shoot him.
"It started as a joke years ago that there was going to be a line in the first season where we were sketching in the Holmes family," Gatiss said in a statement. But we thought we wouldn't do much and thank god we didn't because it has given us this place to go."
The upcoming season finale is said to be the real story that the TV series has been telling from the very beginning, and fans will surely be in for a special treat as there are no hints from show executives as to how the new installment will come to an end.
The final episode of "Sherlock" season 4 will air on Sunday, Jan. 15, 8:30 p.m. on BBC One.