“The roads we walk have demons beneath and yours have been waiting for a very long time,” the preview begins with a voiceover.
Toby Jones will play the villain in this season, and in the teaser he seems to exert power over Cumberbatch’s Holmes. “What’s the very worst thing you can do to your very best friends?” the evil man asks. “Tell them your darkest secret.”
After several quick glimpses showing many of Holmes’ dearest friends, the camera cuts to a scene of the main character staring straight into the camera: “I love you,” Holmes says.
The BBC series was created by Steven Moffat and Mark Gatiss, and also stars Martin Freeman as John Watson.
In an interview with British GQ, Cumberbatch hinted that this time around might be the last for the hit program. “It might be the end of an era. It feels like the end of an era, to be honest. It goes to a place where it will be pretty hard to follow on immediately,” he said.
The fourth series of “Sherlock” will premiere on Jan. 1, 2017.
Watch the “Sherlock” Season 4 preview below: