"Sherlock" Christmas special episode could be based on Arthur Canon Doyle's story "The Adventure Of The Blue Carbuncle." Though there is no confirmation of the plot being based on Blue Carbuncle, what makes the speculation go rife is the fact that it is a lighthearted story laced with the theme of Christmas.
Moreover, in the previous season fans were also given a sneak peak of the story when in the episode titled "The Empty Hearse" Mycroft Holmes and Sherlock Holmes play the game of identities to solve the mystery of a Christmas hat abandoned on the steps of 221 Baker Street.
Doyle in Blue Carbuncle describes this particular scene when Sherlock discovers the Christmas hat and dead goose at his doorstep. Further in the story Sherlock sends the goose to be cooked and plays identity game with Mycroft. Later Sherlock discovers that the goose contains a jewel, which belonged to Countess of Morcar's Blue Carbuncle. Sherlock and his partner in crime Dr. James Watson then start solving the mystery of hidden jewel in the goose.
There is a possibility that this could be the story of "Sherlock" Christmas special as Mark Gatiss has hinted about this particular story earlier.
"It was great, we leaked some photographs of Baker Street at Christmas [for A Scandal In Belgravia] and everyone then assumed it was The Blue Carbuncle", he had said in his interview with BBC earlier.
Meanwhile, Martin Freeman said that "If that's going to be a special - I'm speaking off-message here; if this was New Labour I'd get fired - I think that might be for next Christmas. A Christmasspecial. That's what I understand," reported Telegraph.
Freeman further suggested that the show makers' try to follow the original stories by Doyle without much deviation. "While we play fast and loose with the original stories, we generally follow the trajectory of what Conan Doyle did. So he [Watson] gets married, and then Mary dies - so at some point, presumably she'll die," he noted.
Also, post "Sherlock" Christmas special; fans would get their hands on season 4. The new season has been confirmed by the show's co-creator Steven Moffat and it would pick up two years after season 3, as per the reports.
"We're reasonably confident that the very next thing to happen to Sherlock and John, is the very last thing you'd expect," Moffat told BBC
"Series Four! At last! It's always special to return to Sherlock but this time it's even more special as we're doing... a Special!" co-creator Mark Gatiss (who plays Mycroft) said in a statement about season 4.