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"Sherlock" fans warned to expect "tragedy" in season 4

“Sherlock” stars Martin Freeman and Benedict Cumberbatch.

BBC One

Hold onto your deerstalker hats, "Sherlock" fans -- there will apparently be some harrowing moments to come when the show returns for season 4.

"You can always expect tragedy as well as adventure, that's just how it goes," series co-creator Mark Gatiss said in a recent interview with RadioTimes.com.

It was announced in July that "Sherlock," starring Benedict Cumberbatch as the titular supersleuth and Martin Freeman as John Watson, would be returning with a new special episode, to be followed by a three-episode fourth season.

Gatiss didn't say what that tragedy would be, but one theory involves John's wife, Mary, played by Amanda Abbington. As Freeman noted in a June interview with Britain's Telegraph, "While we play fast and loose with the original stories, we generally follow the trajectory of what Conan Doyle did. So [John] gets married, and then Mary dies -- so at some point presumably she'll die."

But still, Gatiss warned viewers, "Just because it's in the stories doesn't mean it'll happen in the series because there's an awful lot of changes and an awful lot of places to go and things to do. It should be clear by now that while, of course, Doyle is our absolute god, we have gone quite a long way away as well ... so there are lots of surprises to come."

Filming on the special episode of "Sherlock" is due to begin in January, while shooting on season 4 is slated to happen later that year.