The BBC and the official Doctor Who Twitter feed set dual fandoms buzzing this afternoon with word that Game Of ThronesMaisie Williams, aka Arya Stark, will do a guest turn on the next season of the cult series. Today is Williams’ first day on set — see pic of her on the TARDIS above. Although her role is being kept under wraps, lead writer and executive producer Steven Moffat did offer that “she is going to challenge the Doctor in very unexpected ways. 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.” The news comes as the GOT faithful waits breathlessly for the April 12 Season 5 debut of the HBO fantasy series.

Two Doctor Who episode titles have also been revealed for the ninth season that sees Peter Capaldi return as the Time Lord with Jenna Coleman as companion Clara Oswald. Both period adventures, they are The Girl Who Died, written by Jamie Mathieson and Moffat, and The Woman Who Lived by Catherine Tregenna.

Also joining the guest cast are Rufus Hound, Tom Stourton, Ariyon Bakare, Simon Lipkin, Ian Conningham, Murray McArthur, Barnaby Kay, John Voce, and Struan Rodger.

Doctor Who returns to BBC One and BBC America in the fall.