Doctor Who: Battlefield

A squad of UNIT troops is escorting a nuclear missile through the English countryside, whilst nearby knights in armour are fighting battles with broadswords, guns and grenades.

The Doctor arrives on the scene and meets two old friends: Brigadier Lethbridge-Stewart and Bessie, the souped-up Edwardian roadster.

Why do the knights address him as 'Merlin', and what is the power of the sword that Ace retrieves from the bottom of a lake? Will the wicked Morgaine carry out her threat to destroy the world?