Everything Sherlock fans know will be "tested" as the detective thriller series returns to screens at the beginning of next year.
A first trailer for show's fourth season, broadcast on BBC1 on Wednesday, revealed that it will begin on January 1 2017.
The teaser, featuring Benedict Cumberbatch as the contemporary adaptation of Sir Arthur Conan Doyle's Sherlock Holmes, gave glimpses of upcoming scenes with the mysterious lines: "Everything they know will be tested...Everyone they know is under threat."
It comes the day after Cumberbatch himself hinted that the next series will be so dramatic fans might require a break from the show afterwards.
But the British star, 40, revealed there could be a longer gap than normal between instalments after the forthcoming episodes.
He told the Press Association on Tuesday: "I think there will be a lull. I don't think it will be in a year and a half necessarily but we will wait and see and maybe it will be.
"First and foremost we have got a fourth series for people to watch and understand and when they watch it they might go 'Oh yeah, maybe there does need to be a break now' because of what happens in it. No spoilers!"
However, Cumberbatch was adamant that his new role as superhero Doctor Strange in Marvel's new film of the same name will not impact on his future in Sherlock.
He said: "Let me put that one straight, that is nothing to do with this.
"They moved the schedule to accommodate Sherlock, there is a lot of mutual love between those two enterprises and there is no reason why one cancels out the other."
The comic book giant was so keen to cast Cumberbatch in the blockbuster movie they shifted the filming schedule and release date, he said.
"You have a lot of responsibility, especially to them, because in this instance they did something they had never done before because I was doing Hamlet (at The Barbican in London) and that is when they wanted to film after offering me the role and I had to turn it down.
"They went to other places with the part but kept coming back to me and eventually went 'Oh all right, we will change our filming schedule and release date' which is a huge, huge honour."
Doctor Strange is in UK cinemas now.