Alistair Gordon Lethbridge-Stewart, often simply known as the Brigadier, was the head of the UK branch of UNIT, the United Nations Intelligence Taskforce.

He first met the Doctor when, as a Colonel in the ordinary army, he became involved in fighting against the Great Intelligence invasion of London, via the London Underground.

His experience in fighting the Yeti let him to be appointed head of UNIT as the Earth braced itself against future extra-terrestrial invasion. He met the Doctor for the second time when the Cybermen attempted to invade London

His longest working relationsship with the Doctor came when the Doctor was forced to regenerate and was exiled to Earth. Although a first suspicious of the Doctor's new appearance he soon grew to trust and respect the man and they had a very long and close relationship, appointing the Doctor as UNIT's Scientific Adviser. Their few disagreements were over the Brigadier's willingness to use force against the enemy while the Doctor preferred to negotiate. 

Together they fought Daleks, Daemons, the Master and a succession of monsters. The Brigadier was there when the Doctor's body, ripped apart by the radiation on Metabenus III, regenerated into the form of the fourth Doctor.

The Brigadier eventually left UNIT and obtained a job as a master in a minor public school. It was here that he met the Doctor once more when he became involved  Mawdryn attempts to steal the Doctor's remaining regeneration's.

Following his marriage to his life long love, Doris, he had his final meeting with the Doctor, when he was called out of retirement to deal with an invasion of armoured knights led by Morgaine.  Lethbridge-Stewart faced down and killed the demonic Destroyer of Worlds armed only with his service revolver and a load of silver-tipped bullets

Although the Brigadier never met the Doctor again, he continued his work for UNIT into the 21st Century, undertaking many missions to Peru. He was knighted for his work with UNIT. He was involved with his former UNIT colleague, Sarah Jane Smith, when he helped her in accessing UNIT's "Black Archive", a top secret alien artefact facility.

The Brigadier died peacefully in a nursing home. The Doctor learnt of his death while ringing the home hoping to speak to his old friend during the events leading up to The Wedding of River Song