When Doctor Who finally returned earlier this month after more than year off the air, it was with a delightful new companion and a renewed sense of purpose, a rather spectacular feat for a series that celebrated its 50th anniversary a few years ago.

While it's true there's always a sense of excitement when someone new joins the long-running British sci-fi series about an alien who can travel through time and space, the addition of Pearl Mackie as the the highly inquisitive and immediately likable Bill Potts has given the show a much needed breath of fresh air.

The series is also operating with a much a lighter tone after a rather dramatic season in the TARDIS that saw the Doctor lose and then save his previous companion Clara Oswald (Jenna Coleman), only to ultimately forget his memories of her altogether. Current showrunner and executive producer Steven Moffat, who will depart the series alongside star Peter Capaldi after this season, says this new tone and direction is the natural result of the show starting its well-traveled loop over again and following Bill and her emotional journey.

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"The beginning of knowing the Doctor is the most exciting period of that fictional character's life ever. It's incredible," Moffat tells TVGuide.com. "You can revisit this huge period of discovery that, while you thought your life was boring day after day and you were just serving chips, that actually, if you fall through this trap door, there's an absolute magical, mystery alternative to all the mundanity of your normal existence."

"For that reason, because we get to start that story again, it also centers it on the least dark character," Moffat continues. "It's [Bill's] drumbeat rather than the Doctor's that the show follows."

In Saturday's new episode "Thin Ice," Bill — a Black woman and the show's first openly gay companion — and the Doctor travel to Regency-era London, where a mysterious creature has taken up residence under the frozen Thames. Among other things, the hour will directly address race because, according to Moffat, the writers "didn't see an alternative."

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"History is always white washed," says Moffat. "How do we manage to have a diverse cast despite that? The way that we did it was ... [to just] say that you will see people of different colors there. In fact, there were. People all didn't arrive in the twinkle of an eye. It is bending history slightly, but in a progressive and useful way."

"Also, it wouldn't be a pleasant place for [Bill] in several respects. Taking that on is just respectful of the audience really," he continues. "It's a chance to — I'm always reluctant to sound so pious and so do-gooding and all of that. It's useful that these things are talked about. The evil in Doctor Who can sometimes be the evil in our real world, too."

Find out what evil awaits them when Doctor Who airs Saturday at 9/8c on BBC America.

Additional reporting by Alex Zalben