Clara is floating in space, crying out to the Doctor for help. The Doctor is tied up in some kind of battle in the TARDIS. Clara has some kind of creature in her spacesuit. She describes her surrounding to The Doctor. She screams as she finds herself onboard the TARDIS, the Doctor tearing off her helmet and killing the creature inside by stomping on it. The Doctor walks outside to wipe his feet on the grass. Clara follows him, and they find themselves surrounded by Vikings. The Doctor tries to intimidate them with his sonic sunglasses, but one of the Vikings simply takes them from his face and snaps them in half.
The Doctor and Clara are taken back to a town. The group escorting them in are greeted with much applause and enthusiasm from the townspeople, particularly a young girl named Ashildr, who says she dreamed they all died. Ashildr and the Doctor lock eyes as they pass. Clara asks the Doctor if she knows the girl, but he says he’s never seen her before I his life. The girl look at something in her hand.
The Doctor and Clara are presented to the town leader. The Doctor slips out of his shackles and claims to be Odin, calling a Yo-Yo his sign. The villagers tell him he’s not Odin. The Doctor asks if they’ve ever seen Odin, and just then Odin’s face appears in the sky. Odin says the warriors will feast in Valhalla tonight, and sends mechanical soldiers. The soldiers, called the Mire, begin choosing villagers to harvest. Clara has Ashildr use a piece of the sonic sunglasses to protect the villagers, but they are taken.
Ashildr, Clara, and several Vikings find themselves in “Valhalla.” One Viking leads the way, but some sort of power cascades through the chamber and kills him. The walls begin moving to push the remainder into the energy chamber. The Vikings try to hold the wall, while Clara and Ashildr try to open the door on the other side.
The Doctor is in the village with the remainder of the townsfolk. The Doctor tries to convince them that the Odin who appeared is false, that that they were raided.
Clara wakes Ashildr and tells her to be quiet. Ashildr wonders why they’re still alive. Odin and two Mire appear and tell her that it’s because of the sonic sunglasses, a piece dangling from his hand. Clara apologizes for scaring Odin. Another mire appears, bringing Odin a vial of testosterone extracted from the finest warriors. Clara asks if this is a war Odin really wants. Ashildr steps in, and challenges Odin to a battle. Odin agrees to a battle between the village and ten of his warriors. Clara asks why he’s doing this and Odin says for the joy of war, then shows her his true, alien face.
Clara and Ashildr are returned to the village. The Doctor is pleased, assuming Clara convinced the Mire to leave. Clara explains that Ashildr declared war on them. Ashildr apologizes. Her father explains that she thinks she brings the village bad luck. The Doctor tries to convince the remainder of the townsfolk to leave the village, but they refuse. The Doctor tries to walk away, but Ashildr stops him. He keeps going.
Clara finds the Doctor outside. He says it’s just one village. If they defeat the Mire, then it could attract unwanted attention to Earth. Clara asks the Doctor to translate a baby crying, and he decides to stay. The Doctor begins trying to train the villagers.
The villagers eat. Outside, the Doctor and Clara listen to the weapon forges of the Mire spinning up. Clara says training villagers to fight isn’t very like him. He says he used to believe that before he met Clara.
Inside her tent, Ashildr beats a puppet Odin with a stick. The Doctor finds her there. He asks her what she thinks their changes are tomorrow. She says every single one of them is going to die. Ashildr explains that she has always been strange, and different from the others, but here she is loved. She tells him that leaving this place would be death itself. The Doctor hears the baby crying again and remembers something she cried.
The Doctor runs to the boathouse, having realized what he missed. He’s filled with confidence, and promises Ashildr they’ll never have to leave the village. They have electric eels in the water. They prepare through the night. The Doctor tells them getting a helmet is the key, and they’ll use Ashildr’s monstrosity.
Odin and the Mire arrive for the battle the next day and find the villagers dancing in the town hall. The Doctor tells Odin that they’re unarmed, but Odin says he’s fought civilians before. The villagers spring their trap, using rigged electronics to shock the Mire and magnets to pull their helmets off. The Doctor takes on of the helmets and Clara grabs one of their weapons. The doctor brings the helmet to Ashildr. She puts it on and reams up a giant snake creature to attack the Mire. The Mire retreat. Ashildr stops pretending, and the snake is revealed to be a puppet. Clara recorded the whole thing, so the Doctor threatens to show the embarrassing episode throughout the galaxy if the Mire doesn’t retreat now. Odin promises he and the Doctor will meet again, but the Doctor sends him teleporting away. The Mire’s ship departs.
Ashildr’s father goes to congratulate her, but she doesn’t respond. They pull the helmet off of her, but she has no pulse.
Clara finds the Doctor. He says the machine used Ashildr up like a battery. He says he’s sick of losing people. He laments not being able to make enough waves to save her. The then catches a glimpse of his own face in the water and remembers Donna, remembers the Tenth Doctor, and the adventure to Pompeii where he first saw his own face. He realizes he chose this face to remind him that he’s the Doctor and he saves people.
The Doctor reprograms Mire tech to save Ashildr. It becomes part of her and saves her. He leaves a second piece of tech behind.
Returning to the TARDIS, the Doctor explains that the tech will never stop fixing Ashildr. She’s functionally immortal. He left the second kit in case she finds someone she can’t bear to lose. The Doctor is feeling some regret, afraid this his emotions led him to making a mistake. He realizes Ashildr is now a hybrid.
Time passes around Ashildr.