The Doctor and Clara have arrived at a 9th century Viking village, but unusually the Vikings aren’t the most bloodthirsty race running riot.
A ferocious mechanical alien army, led by a treacherous Viking ‘god’, Odin, has arrived there, too, and they attack everything they come across. These are the galaxy’s deadliest mercenaries, The Mire, and most of the village’s Viking warriors are slaughtered or imprisoned.
One brave young Viking girl, Ashildr (Game of Thrones star Maisie Williams), refuses to be beaten, however, and foolishly decides to declare war on The Mire. Their leader Odin loves a challenge and says he’ll give her village a sporting chance – the remaining few villagers can have a day to prepare themselves before The Mire return to finish them off.
There’s one huge problem for the village in their impending battle against the aggressive aliens. The only people left alive are shepherds and farmers who aren’t at all warrior-like. None of them have even held a sword! So the Doctor and Clara have a day to turn them into a fighting force capable of facing The Mire.
Of course, Ashildr is the most eager to see the action but, as the Doctor goes about training the Vikings, he seems to recognise Ashildr from somewhere else. He’s seen her before, but can’t quite place her. What’s the big mystery?
Soon the pair has a heart-to-heart and from what Ashildr reveals, the Doctor tries to discover who she really is…
The battle goes well for the Vikings, with the Doctor’s cunning help, but alas Ashildr dies…. only to be revived soon after by the Doctor who uses a spare piece from one of The Mire and puts it inside her! He tells Clara that Ashildr is now a hybrid and functionally immortal… Will she be seen again on their time travels?