Maisie Williams is back on Doctor Who, but is she a friend or a foe?

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Centuries after Twelve (Peter Capaldi) pulled Ashildr back from behind the veil, the little Vikings is honing her highway robbery skills in the 17th century. At the same time, a bandit called "The Nightmare" and his/her monstrous, fire-breathing sidekick are terrorizing the streets of London. Is Ashildr wrapped up in The Nightmare's reign of terror? Either way, Twelve will be on the case.

"It's always nice to get to do a historical episode and we get to go to one of my favorite periods, which is the 17th century," Capaldi tease in a preview. "It's all the fun of highwaymen but also there's a stranger and more cosmic mystery at the heart of it which sort of pushes the Doctor into a tight corner. It's clear that the events that happen in this episode will have a bearing on his future."

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Did the Time Lord make a devastating mistake when he saved Ashilr, or will she be integral to keeping the Earth from an impending betrayal? Find out when Doctor Who airs Saturdays on the BBC and BBC America.