“Sherlock” Season 4 might have to borrow the TARDIS from “Doctor Who” in its coming special. It seems Sherlock (Benedict Cumberbatch) and Watson (Martin Freeman) will be headed back to the Victorian era. While the original Arthur Conan Doyle books were set back then, the PBS series has thus far been a modern take on the classic.

Showrunner Steven Moffat revealed the time-traveling twist at SXSW last weekend and also noted that the Victorian special would not be part of Season 4. Instead, the 90-minute special will be a stand-alone episode.

“The special is its own thing,” Moffat told Entertainment Weekly. “We wouldn’t have done the story we’re doing, and the way we’re doing it, if we didn’t have this special. It’s not part of the run of three episodes. So we had this to do it – as we could hardly conceal – it’s Victorian. [Co-creator Mark Gatiss] and me, we wanted to do this, but it had to be a special, it had to be separate entity on its own. It’s kind of in its own little bubble.”

How exactly will Sherlock and Watson travel through time on the Emmy award-winning series? Entertainment Weekly assured audiences that it wasn't a dream sequence or costume party, but we have a few other theories:

1. Cold Case – Sherlock may be picking up a cold case from the Victorian era. If that happens, he might have to picture himself then to solve the crime.

2. Wormhole – Could one of their cases lead them to finding a mysterious vortex or wormhole that throws them back in time? Granted, that's a bit more sci-fi than the mystery series typically is, but this storyline is quite unusual.

3. Drugs – Time travel sounds pretty trippy, so maybe this is a drug trip. Sherlock and Watson don't typically take recreational hallucinogens, but perhaps one of them will be drugged by an enemy.

4. The TARDIS – Moffat is also the showrunner for “Doctor Who.” Is this the "WhoLock" crossover we’ve all been waiting for?

Fans will have to wait until the series returns to find out why the sleuths are headed back in time. No one knows exactly when that will be, however. “I think that might be for next Christmas. A Christmas special. That’s what I understand,” Freeman told the Telegraph last year. No official announcements have been made about the return date yet.