Two Doctors stranded in a forbidding snowscape, refusing to face regeneration. And a British army captain seemingly destined to die in the First World War, but taken from the trenches to play his part in the Doctor's story. This is the magical last chapter in the Twelfth Doctor's epic adventure. He must face his past to decide his future. And the Doctor will realise the resilience of humanity, discovering hope in his darkest frozen moment.