Actor Toby Jones joins the cast of "Sherlock" for season 4 and he will play the role of Culverton Smith.

Culverton Smith is a fictional character created by writer and physician Sir Arthur Conan Doyle who appears in "The Adventures of the Dying Detective" series of Sherlock Holmes short stories. In the story, Holmes has contracted an illness that alarms his landlady after three days of seemingly nearing his death. She calls Dr. Watson who comes to see Holmes and worries as his friend refuses to be attended to. Instead, Holmes asks his friend to look for Smith who is not a physician but an expert on the disease he has contracted. Before Smith arrives, Watson is asked by Holmes to hide, in order to listen to their conversation.

It turns out that it is Smith who intended to get Holmes killed. He goes on to confess to murdering his nephew by inflicting the same disease that infected Holmes. He also reveals to have a grudge against the detective who figured out the crime he committed. In the end, Holmes gets out of bed and reveals it was all part of his plan and he was never sick.

Season 4 is said to be loosely based on the investigation against Smith and how Holmes (Benedict Cumberbatch) outsmarts the villain. However, it is believed that the character is part of the big reveal, which many believe in the comeback of Jim Moriarty (Andrew Scott) who is known to be Sherlock's greatest nemesis.

Co-creators, co-writers, and executive producers Steven Moffat and Mark Gatiss have revealed that season 4 will lead back to Baker Street. They also teased that aside from returning to the familiar place, ghosts of the past will haunt Holmes and his friend and staunch ally, Dr. Watson (Martin Freeman).

The return of Moriarty was hinted on the special, "The Abominable Bride." Filming for season 4 has reached Morocco and the location is believed to be connected to Moriarty's story arc. Another story to look forward to in season 4 is the birth of Dr. Watson's child and a possible love interest for Holmes.

No official announcement has been made regarding season 4's premiere, but it is widely believed to return no later than January of 2017.