Almost 20 years after his last appearance, the character returned in the spin-off series, The Sarah Jane Adventures in late 2008. Previous to this, his portrait was seen hanging in Sara Jane Smith's attic in 2007's "Invasion of the Bane".
Alistair Gordon Lethbridge-Stewart was born in Scotland, according to dialogue in Terror of the Zygons. He first encountered the second Doctor (Patrick Troughton) in The Web of Fear (1968), when he appears as a colonel commanding a British Army detachment sent to investigate the Yeti in the London Underground. By his next appearance in The Invasion (1968), he had been promoted to Brigadier and was working with UNIT. When the Doctor was forcibly regenerated and exiled to Earth, Lethbridge-Stewart gave him a position as UNIT's scientific advisor. Other military members of UNIT included Captain Mike Yates, Sergeant Benton and Royal Navy Lieutenant Harry Sullivan.
In his initial appearances, Lethbridge-Stewart was portrayed as a stereotypical by-the-book Martinet. Very often, the Doctor felt frustrated at working with him because the Brigadier's typical response to any threat was to shoot at it; a well-known phrase of his was, "Five rounds, rapid." The Brigadier's decision to destroy the Silurians' base, despite the Doctor's continuing efforts to reconcile the reptiles (who had just made two credible attempts to exterminate Humanity and were still openly determined to achieve this goal) with human beings, in particular, drew the pair into conflict. In turn, Lethbridge-Stewart was sceptical of the strange phenomena and super science the Doctor habitually encountered, and just as frustrated with the Doctor's eccentricities. However, over the years the two developed a close working and personal relationship as well as mutual respect for each other's abilities.
The Brigadier always faced the unknown with unflappable British aplomb. He has shown himself to be a true warrior in combat, ruthless when he has to be, and heroic in the face of the often overwhelming odds that he and UNIT faced over the years. He eventually retired from the military to teach mathematics at a British public school in 1976, as seen in Mawdryn Undeed (1983).
Most of the Third Doctor stories were set on Earth and feature UNIT and the Brigadier heavily. While not as ubiquitous in the following years, he appeared alongside every subsequent Doctor in the original television series run with the exception of the Sixth Doctor. Although Lethbridge-Stewart first met the Doctor in his second incarnation, he also met and worked with the First Doctor in the 20th anniversary special, The Five Doctors. The Brigadier and the Sixth Doctor, as well as later incarnations of the Doctor, have been paired in numerous spin-off productions
As one of the most popular recurring supporting characters in Doctor Who, the Brigadier is often listed among the Doctor's companions. He is listed as such by the BBC and included in John Nathan-Turner's (a former producer of Doctor Who) book discussing all of the Doctor's companions.
Lethbridge-Stewart's last appearance in a Doctor Who television episode was in 1989, in the Seventh Doctor serial Battlefield. Called out of retirement to deal with an other-dimensional invasion of armoured knights led by Morgaine, he found himself once again at the Doctor's side. Lethbridge-Stewart served as his world's champion as he faced down and killed the demonic Destroyer of Worlds armed only with his service revolver and a load of silver-tipped bullets. (Battlefield was stated to be a few years into Ace's future but not a specific date. The Virgin New Adventures books place it in 1997.)