Let's play detective. We spy goosebumps, quickening pulses, and fans sitting on the edge of their seats. Elementary, my dear Watson: Someone's just watched the season 4 trailer for Sherlock The beloved BBC crime drama won't return until 2017, but San Diego Comic-Con delivered a little preview with the release of the trailer, below. Season 4 is going to get dark and edgy. Look! Watson's (Martin Freeman) hair turned white!Toby Jones and Lindsay Duncan appear in the trailer, which sees Benedict Cumberbatch's Sherlock encountering enemies new and old. Moriarty pops up (somehow), Mrs. Hudson talks tough, and, as the video teases, everyone is under threat. The Hollywood Reporter reports that Amanda Abbington, who plays Mary, Watson's wife, told the crowd, "It's really dark...it’s the darkest that Steven [Moffat] and Mark [Gatiss, who also plays Mycroft] have written. When we read them, all of us, we were kind of overwhelmed by them, because they were shocking and amazing, as always."