'Sherlock' season 4 episode 3 spoilers: 'The Final Problem' to bring a long-buried mystery to an explosive conclusion

From one shocking end to another, BBC's "Sherlock" sure isn't pulling any punches in its fourth season as it brings in an adversary like no other.

The previous episode of "Sherlock" has concluded with reconciliation and forgiveness between friends and partners-in-crime-solving, Sherlock (Benedict Cumberbatch) and Watson (Martin Freeman). However, could this reunion be cut short by the arrival of a previously unknown Holmes sibling?

When John's therapist, Elizabeth (Sian Brooke), confessed to disguising herself several times to get close to Sherlock and Watson, then revealed herself to be the Sherlock's "secret sister," it seemed to have set the tone for a rather deadly encounter coming up in the final episode of "Sherlock" season 4. Such a scenario is probable, especially after the pretend-therapist, whose real name is Eurus, pointed a gun at Watson's face, intending to blow a hole in it.

The upcoming third and final episode of "Sherlock" season 4 will finally bring Sherlock face-to-face with the sister whose memories have been steering Sherlock's life choices to this day, as what Mycroft (Mark Gatiss) tells him in the episode promo. Whatever it was that Eurus has done may be made clear after everything falls apart and long-buried secrets finally find their way back to the surface.

"I know this is difficult, but you have got to keep it together," Watson tells his friend in the trailer as they both gear up to face what could possibly be the greatest, most dangerous challenge they would ever have to face together. Was Eurus the one sending those "Miss Me?" messages all along? What does she hope to accomplish by playing these mind games with the Baker Street duo?

In an interview with Radio Times, Freeman shared that the unexpected turns that the fourth season has and will still be taking also shocked him as much as they shocked the fans of the series.

"There are a few moments in this series that I couldn't believe," Freeman said. "It was making me go cold reading it," he went on to say.

Freeman also claimed that if they did everything right by the script, these season's episodes will go down in the books as the show's best episodes yet.

The season finale of "Sherlock" season 4 airs on Sunday, Jan. 15, at 9 p.m. GMT on BBC One. The finale episode, with 15 minutes of exclusive extras, will also hit 350 movie theaters across the United States on Monday, Jan. 16, and Wednesday, Jan. 18, at 7 p.m. local time.