Doctor Who: Dark Horizons

On a windswept Northern shore, at the very tip of what will one day become
Scotland, the islanders believe the worst they have to fear is a Viking attack.
Then the burning comes, consuming everything in its path — yet the burned still speak. The Doctor encounters a people under attack from a power they cannot possibly understand. They have no weapons, no strategy, and no protection against a fire sent to engulf them all. The islanders must take on a ruthless alien force in a world without technology; without communications; without tea that isn't made out of bark. Still at least they have the Doctor on their side
— don't they?