In a forgotten corner of history, a young Viking girl sits surrounded by the dead warriors of her people. She wants to fight back against the perpetrators – and, in doing so, declares war on the Mire, one of the deadliest races that the galaxy has ever known. The Doctor and Clara must help protect her village, using the twelve tense hours they have to turn a motley crew of farmers and blacksmiths into a fighting force to be reckoned with.