After saving Clara from near death, the Doctor lands in medieval Europe with Clara, and are almost immediately captured by Vikings. Taken to the village, they are unexpectedly greeted by an image of Odin-who then captures all the Viking warriors as well as Clara and a Shield-maiden, Ashildr.
“Odin” however is an alien villain-The Mire-who then kills the warriors and ‘drinks’ their adrenaline as a form of nectar. Although Clara tries to find a way out of the situation, Ashildr declares vengeance for her fallen warriors. The Mire gives them 24 hours to prepare.
Problem is, all the town’s warriors are dead, leaving just the civilians who have no experience in combat. The Doctor attempts to train them. It doesn’t go well.
Eventually, he devises a plan B. It turns out the town has some electric eels in storage, and also the imagination of Ashildr-a storyteller. He uses the eels to short circuit and remove a mire suit, and then puts the helmet on Ashildr, using her imagination to conjure a holographic dragon that scares the Mire into retreat. The whole thing is captured on cellphone video, humiliating the Mire. Unfortunately, this proves too much for Ashildr’s body and she dies.
Feeling defeated, the Doctor retreats to the barn where the eels are stored. He looks into a water barrel, and suddenly remembers why his face looks so familiar. It’s the face of the Roman father he rescued from Pompeii in his tenth incarnation-and when he regenerated, he subconsciously took a face that resembled him as a reminder-that he can always save people-no matter the cost.
He’s able to use the Mire’s technology to bring her back to life, although this now makes her immortal. As they leave her alive, the Doctor wanders what will become of her-as she outlives all her loved ones?
He’s about to find out. Running to her centuries later while on the trail of an artifact, she has become bitter after centuries of losing loved ones, and is also looking for the artifact-one capable of opening a portal to another world. She wants to get off Earth and travel with the Doctor-but the Doctor refuses.
Turns out-as “Lady Me”-a name that she’s taken because of her loneliness-that she’s actually working with a cat-like alien, who wants to use the artifact to open the portal, helping her to escape Earth. It requires death to activate; although Me initially wants to use her butler, she instead chooses to use a rival Highwayman, Sam Swift, who is being sent to the gallows.
Although the Doctor is able to stall for time using Gallows humor and a ‘pardon’ using his psychic paper, Me kills Swift by bonding the artifact to him. However, it turns out the alien really just wants the portal to open the way for an invasion fleet; as people start to get killed by the fleet, Me suddenly regrets her decision and brings Swift back to life with a Mire chip, stopping the invasion. Talking to the Doctor later on, Me says she’ll watch out for his companions on his behalf-a promise she makes clear when she appears in the background of Clara’s selfie after she reunites with the Doctor….