Someone is bound to not make it to the wedding of Watson and Mary, and it is the duty of bestman Sherlock to stop it.
The wedding day is finally here, but true to "Sherlock" nature, it is not going to be an ordinary one. In "The Sign of Three", Watson marries journalist Mary Morstan, played by Amanda Abbington who is Martin Freeman's partner in real life.
The synopsis says, "Sherlock faces his biggest challenge of all - delivering a Best Man's speech on John's wedding day. But all isn't quite as it seems. Mortal danger stalks the reception - and someone might not make it to the happy couple's first dance. Sherlock must thank the bridesmaids, solve the case and stop a killer.
Abbington told Mirror back in October that her character is to be the third wheel. "They [Sherlock and Watson] do have this beautiful relationship, so getting in between that - which is what Mary does, she becomes this kind of third wheel - was scary," she said.
The actress continued, "Also, the fans love these two together, so I'm sure Mary won't be particularly liked by them, but I hope that on the whole people really like her, because she's a great character and she has some fantastic secrets."
"The Sign of Three" airs January 5 on BBC One. Over in United States, "Sherlock" Season 3 will premiere on Masterpiece on PBS on Sunday, January 19, 2014 at 10 P.M.