Peter Capaldi (Twelfth Doctor)
Jenna Coleman (Clara Oswald)
Maisie Williams – Ashildr
David Schofield – Odin
Simon Lipkin – Nollarr
Ian Conningham – Chuckles
Tom Stourton – Lofty
Alastair Parker – Limpy
Murray McArthur – Hasten
Barnaby Kay – Heidi
Directed by Ed Bazalgette
Written by Jamie Mathieson
Script editor Nick Lambon
Produced by Derek Ritchie
Executive producer(s) Steven Moffat
Incidental music composer Murray Gold
Series Series 9
Length 45 minutes
Originally broadcast 17 October 2015
The TARDIS lands in a Viking village, and the Doctor and Clara find themselves surrounded by Viking warriors, who are impressed by the Doctor's claims of being Odin and his magic. Suddenly, a figure of Odin appears in the sky, offering to take the warriors to Valhalla, and a squad of armored suits materialise, shooting the warriors with weapons that appear to disintegrate them. Clara and Ashildr, a woman from the village, are also struck. The squad soon disappears, leaving a distraught Doctor with the rest of the villagers.
Clara and Ashildr find themselves on a spacecraft with the other Vikings. The men are killed and drained of their adrenaline and testosterone, while Clara and Ashildr meet Odin, the leader of the Mire species that pride themselves on their merciless conquests. Before Clara can stop, Ashildr declares war on the Mire, and Odin grants them 24 hours to prepare. On Earth, Clara brings the Doctor to speed. He recognises the villagers are too weak to fight, and devises a plan using Ashildr's storytelling skills and a supply of electric eels.
When the Mire arrive, the find the villagers celebrating. The Mire's confusion gives the Doctor time to stun them with electricity and pull one of the helmets off with an electromagnet. The Doctor modifies this and has Ashildr wear it, allowing her to envision an articulated puppet as a dragon, which is broadcast to the other Mire and scares them off. Odin vows to attack again, but the Doctor threatens to send video footage of the rout captured by Clara's phone to the universe unless they leave Earth. Odin and the Mire peacefully depart.
The village celebrates its victory until they find Ashildr died from the helmet's use. The Doctor is frustrated until he remembers why he took the form of Caecilius ("The Fires of Pompeii"): to always save someone, no matter what. He "breaks the rules" and modifies two chips from the helmet, once which he implants in Ashildr, and the other he gives to her father. The chip rapidly regenerates Ashildr's body and she regains consciousness.
As they leave, the Doctor tells Clara he fears he gave Ashildr a fate worse than death as the chip will never fail, effectively making her immortal and alone. He provided the second chip in hopes she would give it to one she cares for. The final scene is a time lapse shot of Ashildr watching the world age around her, her expression changing from elation and wonder, to sorrow and misery, and finally hostility.