Maisie Williams
Set in 1651, about 800 years after the previous week's episode "The Girl Who Died", the episode follows the alien time traveller the Twelfth Doctor (Peter Capaldi), while on the trail of an alien artefact that came to Earth, reuniting with Ashildr (Maisie Williams), the woman the Doctor made immortal in the previous episode. Ashildr, having renamed herself "Me", had isolated herself from mortal humans and seeks to use the artefact herself to travel to other planets.