Dr. Who (The Early Years) - Part 73 "Battlefield"

In a darkened Tardis, the Doctor is trying to pinpoint a distress signal. Once landed, the Doctor and Ace discover they are on Earth near a small town called Carbury. As they begin their investigations, they find a nuclear missile convey escorted by UNIT troops, but they have been stopped near Lake Vortigern. Using older UNIT ID's of the third Doctor and Liz Shaw, the Doctor tries to find out more information from the most recently appointed Brigadier, Bambera. She angrily scolds her first officer for letting the Doctor in using such "false" identification.

However, the officer knows something Bambera doesn't. Having worked with Brigadier Lethbridge-Stewart in the past, he recalls the scientific advisor they once had. A scientific advisor named the Doctor who was able to change his persona, appearance and his mannerisms. The Doctor was also well known for the danger he seemed to attract. Bambera puts in a call to Geneva to get what info she can regarding this "Doctor". Meanwhile, at home, the Brigadier is enjoying his retirement with his wife Doris. When she receives a call asking for the Brigadier's assistance, he reminds her he's retired, and as such, not interested. But when he hears the Doctor is involved, he immediately decides he must, much to his wife's reluctance.

Doing his investigation work at a local pub, the Doctor and Ace find an armour clad medieval soldier who goes by the name Ancelyn and identifies the Doctor as Merlin. The Doctor finds out Ancelyn is battling Mordred and his mother Morgaine. The rivalry is actually from a parallel Earth, but when the Doctor meets Mordred, he also identifies the Doctor as Merlin. The Doctor uses this to his advantage and causes Mordred into a retreat. Now somewhat convinced of the Doctor's identity, an unsteady alliance is formed between Bambera and the Doctor, but she's still waiting for the Brigadier to arrive for help.Despite the outward rivalry between Bambera and Ancelyn, it seems they  have begun to fancy each other.

With Ancelyn having "Merlin" on his side, Mordred calls upon his mother Morgaine, who crosses a bridge into our universe. She vows this will be the last stand between her and Arthur. As this is going on, the Doctor and Ace have discovered a space ship under the lake with what appears to be Arthur in suspended animation. The Doctor was able to gain entry when he realized the entrance was keyed to his voice pattern. He then realizes he is most likely "Merlin", but from his own future, most likely a future incarnation of himself.

The Brigadier finally arrives on the scene via helicopter, but Morgaine witnesses his arrival and shoots the 'machine' down. It's then she realizes her son has been fighting on a cemetery of fallen war heroes and claims he has disgraced them and sends him away. Having survived the crash, the Brigadier finds Morgaine but she has called a truce. In her warped sense of honour she refuses to fight on the 'sacred' ground of the dead. She allows the Brigadier to escape, but warns him the next time they meet, she'll kill him. The Brigadier agrees to return the favour and sets off to find Bamera and the Doctor.
[Interesting side note: Nicholas Courtney (The Brigadier) and Jean Marsh (Morgaine) played brother and sister, Bret Vyon and Sara Kingdom, in a first Doctor episode "The Dalek Master Plan"]

As the Doctor and Ace are exploring the underwater spaceship, Ace grabs what is identified as Excalibur. Unfortunately, this triggers a defence mechanism which almost causes her to drown and the Doctor attacked. As Ace is sectioned off from the Doctor, she's ejected from the spaceship and into the lake. She swims to the surface and worried about the Doctor, but the Brigadier has arrived and heads into the underground spaceship and rescues the Doctor. Though not having seen his current incarnation before, the Brigadier immediately identifies the Doctor. Both are glad to see each other once again.

The battle between Ancelyn and Mordred are heating up, casualties on both sides. But when Morgaine stops by the local pub to collect her son, she sees he has had his fill of drinks from the establishment. Her code of honour causes her to pay the 'bar tab' by restoring the sight of the proprietors wife. But thinking "Merlin" could ultimately win, she calls upon "The Destroyer" for leverage. She knows Merlin will withdraw rather than see the Earth destroyed by the "Eater of Worlds". The Doctor tries to convince her though, that Arthur is already dead. In disbelief, she still craves Excalibur and the power it holds.

With the Destroyers help, Morgaine takes Excalibur from Ace. Ace is depressed she let the Doctor down, but he admits he'd rather have her alive, than dead with the sword. The Brigadier then surprises the Doctor with the advancement of UNIT's weaponry to deal with all sorts of invasions. Having silver bullets in stock, the Brigadier kills the destroyer, almost at the cost of his life. Morgaine then decides she's going to use the nuclear missile to destroy the world. But the Doctor calls upon her sense of honour and at the last second she calls the attack off. She's then taken prisoner by UNIT.

Bonus Photo!

The Brigadier surprises the Doctor by bringing with him, Bessie. The car he once drove in his third incarnation when he was UNIT's scientific advisor.

Story Note: This is the final appearance of the Brigadier in Doctor Who, though he is mentioned a few times in the new series.