Doctor Who fans didn't get a female twelfth Doctor, but the Master is a different story entirely.

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The iconic BBC series threw viewers a curveball in the first part of season 8 finale. "Dark Water" reveled that Missy, the Gatekeeper of the Nethersphere, is, in fact, the Master regenerated once more. Showrunner Steven Moffat took viewers behind the landmark decision in Doctor Who Extra.

"I was delighted that the Master came back and doubly delighted that Michelle [Gomez] played the Master, because I know Michelle and knew she'd bring all her customary vigor and wit and sharpness to it," Moffat explained. "She's different from the other Masters we've seen because obviously she's taken a female form."

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According to Moffat, the he has been mulling over the idea to change a Time Lord's gender for some time.

"It's just a proper good old twist," Moffat said. "We've been saying for years now, certainly since I took over, that regeneration can change your gender and that I thought this maybe the one they won't see coming."

Well, he was certainly right about that.

Find out what the Master has planned next when Doctor Who airs Saturday on the BBC and BBC America. Click the video below to see a preview for the second half of the finale, "Death in Heaven."

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Doctor Who, Television, BBC, BBC America