'Sherlock' season 4 news, spoilers: Watson and Mary's new baby creates different dynamic; Sherlock to stumble with childcare?

"Sherlock" season 4 is touted to be the darkest installment yet but that doesn't mean that the crime drama will be devoid of lighthearted moments and storylines. The latest addition to the cast ensures that the upcoming season will still be a lot of fun.

On New Year's Day, viewers will be introduced to the newest cast member who is none other than John Watson (Martin Freeman) and Mary Morstan's (Amanda Abbington) baby. According to BBC, the couple will prepare for their biggest challenge yet — parenthood.

"It creates a different dynamic," co-creator Mark Gatiss told TVLine, adding, "We do have fun with it, because there's fun to be had."

Gatiss added that Sherlock (Benedict Cumberbatch) will treat the baby as a case that needs to be solved but he will have a hard time as "babies don't respond to logic in the same way."

In an interview with BBC, Cumberbatch previewed how Sherlock will react to this new development saying that while his character feels very protective toward the new family, he is not a natural when it comes to dealing with a newborn.

"He is seemingly indifferent which is comic at times but it's all underpinned with a deep love and he's a guardian angel really," the actor said.

Cumberbatch added that while childcare is already quite difficult, it becomes more complicated as the team also has to solve crimes. In the first installment of the three-episode season, "The Six Thatchers," Sherlock waits for Moriarty's (Andrew Scott) posthumous move.

According to the synopsis, Scotland Yard will be baffled by a mysterious case but Sherlock focuses on a seemingly-trivial detail — someone is destroying images of the late Prime Minister Margaret Thatcher. The mercurial detective will try to uncover whether this tied to something more sinister and whether it is connected to Mary's past.

The episode will air on Sunday, Jan. 1, 2017, at 9 p.m. EST on PBS. It will be followed by "The Lying Detective" and "The Final Problem" on Jan. 8 and 15, 2017, respectively.

No word yet on whether there will be a season 5 for "Sherlock."