BBC one unexpectedly dropped a cryptic new teaser for the upcoming season of Sherlock.

The return of the hit series was announced last month with a trailer showing the usually unflappable detective looking particularly up against it.

1 January 2017 is when the next batch of episodes begin, kickstarted with an instalment titled 'The Six Thatchers.'

Benedict Cumberbatch and Martin Freeman both last starred as the detective and trusty sidekick Dr. Watson in 2014, with the latest episode being 'The Abominable Bride' which aired on New Years Day 2016.

The cryptic teaser named 'Rising Tide' shows an animated image of the duo accompanied with the ominous caption: "It's not a game anymore."

According to the BBC, season four will show Cumberbatch’s eponymous character “back once more on British soil as Doctor Watson and his wife, Mary, prepare for their biggest ever challenge - becoming parents for the first time."

This season is widely expected to be Sherlock's last what with Cumberbatch's recent Hollywood-conquering thanks to Marvel film Doctor Strange.