Alistair Gordon Lethbridge-Stewart began his military career in the Scots Guard before ascending to UNIT as a colonel to take part in missions that were of extraterrestrial/ supernatural origin. He would first encounter The Doctor in his second incarnation when they would do battle with the Yeti but would become more familiar with him during the invasion of the Cybermen in the sewers of by which time he had been made a Brigadier.
When the Doctor was exiled to earth and forced to change his appearance by the Time Lords an at first cautious Lethbridge-Stewart was unsure to trust the new Doctor but upon offering him the position of UNIT’s scientific adviser the two would become firm friends battling such foes as The Master, Axons, Daleks and at an unknown point in time humanoid dogs riding giant metal spider machines. With the death of the Third Doctor Lethbridge-Stewart would have two adventures with his successor and would eventually encounter the Fifth Doctor in an event that would see the Brigadier come face to face with the future version of himself, almost causing a disastrous paradox.
He would meet the Fifth Doctor once more along with all of his pervious other incarnations when a time scoop would transport him and several other of the Doctor’s companions to his home planet of Gallifrey to take part in the Game of Rassilon. Some time after retiring The Brigader would take up arms once more to face Morgaine, a sorceress from another dimension along side the Seventh Doctor. In the years that followed Lethbridge-Stewart would receive a Knighthood and would keep strong ties with UNIT occasionally taking part in the odd assignment such as when he became stranded in during an attack on Earth by the Sontaran Empire and even having access to UNIT's “Black Archive”. He would use his position to help Sarah Jane Smith gain access to the “Black Archive” and would even take part in the fight to stop an alien known as the Bane, killing one disguised as a UNIT officer with a gun concealed inside his cane.
Around the time of The Silences pursuit of the 11th Doctor, who had taken to avoiding his seemingly inevitable demise by bouncing around time and visiting his past companions, he contacted the retirement home the Brigadier resided in - in order to go on one more adventure with him only to be informed of his passing some months before which had been peaceful. The news of the Brigadiers death was an instrumental factor in the Doctors decision to return to 1969 and face the impossible astronaut.