Williams, beat known for her role as Arya Stark on the HBO drama series, will play someone who challenges the Doctor "in very unexpected ways," executive producer Steven Moffat teased. "This time he might just be out of his depth, and we know Maisie is going to give him exactly the right sort of hell." Her role is otherwise being kept under wraps.
"I'm so excited to be working on Doctor Who as it's such a big and important part of British Culture," Williams said in a statement. "I can't wait to meet the cast and crew and start filming, especially as we'll be shooting not too far from my home town."
Doctor Who returns this fall.
Find out why you should binge-watch Doctor Who: