Doctor Who Promo - "The Girl Who Died"

Arya Stark playing a Viking fighting an invasion from the future? Yes please!
The Doctor and Clara lead a war against "the Mire," in order to save the Viking warriors who captured them. How did our heroes end up at that Viking village and why does the Doctor take an interest in a brave young Viking girl?
Not to mention, knowing Clara is vacating the TARDIS the title fills me with a sense of foreboding. Is the Doctor's companion "The Girl Who Died" or does the episode name refer to the Viking girl? Could Maisie Williams be his next companion?
Tune in to Doctor Who Season 9 Episode 5 this Saturday to find out.