Alone in the TARDIS, the Doctor (Peter Capaldi) is looking for an alien artefact. He interrupts a highwayman called The Knightmare (voice of Will Brown) in 1651. The victim is Lucie Fanshawe (Elisabeth Hopper), who has the artefact in her luggage, but the carriage drives off before the Doctor can secure it. However, he discovers that the robber is Ashildr (Maisie Williams), the Viking girl he made immortal in the previous episode. In 800 years, she has lost much of her memory, too many loved ones, and re-named herself: Me. The Doctor reads a journal entry about losing her children in the Black Death.
She teams up with the Doctor and they steal the alien artefact from Lucie Fanshawe’s house. They escape by climbing out of the chimney and slip through an ambush set up by Me’s rival highwayman Sam Swift (Rufus Hound). The next day, the Doctor meets Me’s partner Leandro (Ariyon Bakare), a leonine alien stranded on Earth. He can use the artefact to open portals into space. He plans to let Me come with him throughout the galaxy, but to open the portal, someone has to die. Two pikemen (Gruffudd Glyn and Reuben Johnson) announce that Sam Swift is about to be hanged. The Doctor is handed over to them as an accomplice.
The Doctor escapes and pursues Me to the hanging. Me attaches the artefact to Swift’s chest, killing him and opening the portal. Leandro is actually assisting an invasion of the Earth. Spaceships fire on the crowd. Me uses the second long-life chip the Doctor gave her to bring Swift back to life and that closes the portal. Leandro is killed by his people. Me vows to stay and help those the Doctor leaves behind.
The episode was another hit—dark and worldly-wise, visually striking. Near the gallows is a replica of a real handbill concerning the trial of two witches, printed in 1682. The witches are named Rose and Amy. Ashildr once cured an entire village of Scarlet Fever, and the villagers tried to drown her as a witch. The Doctor tells her that the Great Fire of London was caused by Tereleptils, though you could also blame the Doctor for it. He tells her that she might come across Captain Jack Harkness, who is also immortal. Struan Rodger (Me’s butler Clayton) was the voice of the Face of Boe and the three-eyed raven in Game of Thrones. Maisie Williams is quite brilliant in this episode and walks off with it. I think she came close to doing the same in Game of Thrones.