Toby Jones makes an appearance as Sherlock villain Culverton Smith in a new trailer for series four, and it seems that he has the titular hero right where he wants him.

"The roads we walk have demons beneath. And yours have been waiting for a very long time," a voiceover says at the beginning.

"What's the very worst thing you can do, to your very best friends?" Culverton asks. "Tell them your worst secret."

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Co-creator Steven Moffat previously hinted that this would be the darkest season yet, and it looks like that will be the case.

Speaking at a Bafta celebration of the show last week, he said that it "goes to a darker place than I think we've gone before".

Perhaps the most shocking thing of the short trailer is the "I love you" that Sherlock utters at the end - considering the character rarely reveals his emotions, fans will assume that something is up.

Sherlock star Benedict Cumberbatch has suggested that season four could be the last. The first episode will air on New Year's Day at 9am on BBC1.

The season finale, titled 'Sherlock: The Final Problem', is screening at 300 cinemas - including at branches of Odeon, Cineword, Vue and Picture House, around the UK and Ireland "with exclusive content", at the same time that it airs on BBC1 on Sunday 15 January.

Sherlock season 4 in pictures Show all 7 1 /7 Sherlock season 4 in pictures Sherlock season 4 in pictures Mark Gatiss and Benedict Cumberbatch in Sherlock Sherlock, Season 4 premieres January 1, 2017 on MASTERPIECE onÂ PBS. Picture shows: Mycroft Holmes (MARK GATISS) and Sherlock Holmes (BENEDICT CUMBERBATCH) Copyright: Hartswood Films 2016. Sherlock season 4 in pictures Una Stubbs as Mrs Hudson in Sherlock Sherlock, Season 4 premieres January 1, 2017 on MASTERPIECE onÂ PBS. Picture shows: Mrs Hudson (UNA STUBBS) Copyright: Hartswood Films 2016 Sherlock season 4 in pictures Mark Gatiss in Sherlock Sherlock, Season 4 premieres January 1, 2017 on MASTERPIECE onÂ PBS. Picture shows: Mycroft Holmes (MARK GATISS) Copyright: Hartswood Films 2016. Sherlock season 4 in pictures Toby Jones in Sherlock Sherlock, Season 4 premieres January 1, 2017 on MASTERPIECE onÂ PBS. Picture shows: Culverton Smith (TOBY JONES) Copyright: Hartswood Films 2016. Sherlock season 4 in pictures Martin Freeman in Sherlock Sherlock, Season 4 premieres January 1, 2017 on MASTERPIECE onÂ PBS. Picture shows: John Watson (MARTIN FREEMAN) Copyright: Hartswood Films 2016. Sherlock season 4 in pictures Martin Freeman, Amanda Abbington, and Benedict Cumberbatch in Sherlock Sherlock, Season 4 premieres January 1, 2017 on MASTERPIECE onÂ PBS. Picture shows: Behind-the-scenes filming 'Sherlock' Season 4. John Watson (MARTIN FREEMAN), Mary Watson (AMANDA ABBINGTON) and Sherlock Holmes (BENEDICT CUMBERBATCH) Copyright: Hartswood Films 2016. Sherlock season 4 in pictures Amanda Abbington, Benedict Cumberbatch, and Martin Freeman in Sherlock Sherlock, Season 4 premieres January 1, 2017 on MASTERPIECE onÂ PBS. Picture shows: Mary Watson (AMANDA ABBINGTON), Sherlock Holmes (BENEDICT CUMBERBATCH) and John Watson (MARTIN FREEMAN) Hartswood Films 2016

'The Final Problem' is the name of Sir Arthur Conan Doyle's short story where Holmes was meant to meet his end, plunging from the top of the Reichenbach Falls with arch enemy Moriarty. But fans were so devastated that Conan Doyle decided to bring him back.