Head of UNIT
Alistair Gordon Lethbridge-Stewart was a military man, who was put in charge of UNIT when it was first formed. When UNIT started, he had help from the Doctor but the time traveller was never seen on Earth after the 1960s.
During his career, Alistair had to make many tough decisions, including sacrificing Mike Yates to stop the lizard invasion by sending a team back in time to bomb the creatures, initiating the "Plastic Purges" to stop the Nestene, the saturation bombing of Surrey against an unknown threat and losing dozens of men to hold off a Dalek attack. (AUDIO: Sympathy for the Devil, Masters of War)
However, there was rarely any evidence left of the alien threats. As a result, the Brigadier was seen as a dangerous crackpot who caused devastation and economic shortages that he couldn't prove where necessary. Several of UNIT personnel betrayed him, and he claimed that those who made up its ranks were from offices no-one wanted around. He was disgraced, removed from UNIT, and left bitter about his experiences. (AUDIO: Sympathy for the Devil)
By the 1990s, Alistair had moved to Hong Kong (getting stuck there while trying to reach New Zealand), and opened a bar called The Little England. It was on 30 June 1997, the eve of the handover of Hong Kong back to China, that he met the Doctor again. After defeating the Master, the Doctor invited Alistair to join him aboard the TARDIS, an offer which he accepted. (AUDIO: Sympathy for the Devil)
Alistair travelled with the Doctor for a while, until they landed on the planet Skaro. Befriending the Thals, Alistair played an essential part in stopping the Quatch invasion. Because he was reluctant to once again be a military leader, after the invasion was foiled, he chose to stay on Skaro as a peace mediator between the Daleks and the Thals. (AUDIO: Masters of War)