Doctor Who: Battlefield

The TARDIS materialises in the English countryside near the village of Carbury, where a nuclear missile convoy under the command of UNIT's Brigadier Winifred Bambera has run into difficulties. Lying on the bed of the nearby Lake Vortigern is a spaceship from another dimension containing the body of King Arthur, supposedly held in suspended animation, and his sword Excalibur.
Ancelyn, a knight from the other dimension, arrives on Earth to aid the King but is followed by his rival Mordred and the latter's mother, a powerful sorceress named Morgaine. They all recognise the Doctor as Merlin - a fact that the Time Lord attributes to events in his own future.
A battle breaks out between UNIT and Morgaine's men. Brigadier Lethbridge-Stewart has come out of retirement to assist in the crisis and ends up using silver bullets to kill the Destroyer - an awesomely powerful creature unshackled by Morgaine to devour the world - although he himself is almost killed in the process.
Morgaine tries to fire the nuclear missile but is overcome by shock when the Doctor tells her that Arthur is in fact dead. She and her son are then taken prisoner by UNIT.
Episode Endings