Doctor Who: Dark Horizons

On a windswept northern shore, the islanders believe the worst they have to fear is a Viking attack. Then the burning comes. Water will not stop it. It consumes 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; but at least they have the Doctor on their side... Don't they?