The Third Doctor was the third incarnation of the Time Lord known as the Doctor. He was exiled by the Time Lords on Earth for much of his life. The technological limits of the time period often made him frustrated and bitter.
This Doctor worked for the United Nations Intelligence Taskforce as their unpaid scientific advisor. He remained in their service long after his exile was ended. Unlike his earlier incarnations, he was eager to engage others physically. He also cut a more dashing figure.
In his first year, he was assisted by Liz Shaw, whom he displaced as UNIT's scientific advisor. A capable scientist in her own right, she left to pursue her own work. She noted that all the Doctor needed in an assistant was someone to pass him his test tubes and tell him how brilliant he was.
In the field, the Doctor was aided by Sergeant Benton, Captain Mike Yates and even the Brigadier. The Doctor did not get along well with the Brig at first, but they developed an easy mutual trust and became good friends.
After Liz left, the Doctor was given a new assistant, Jo Grant. She was bubblier and had less scientific training. The Doctor found her a most useful companion in his adventures during and after his exile. Jo got in trouble easily and the Doctor became protective of her. She left UNIT to marry Clifford Jones.
In his final year with UNIT, the Doctor was joined by inquisitive newswoman Sarah Jane Smith. They travelled widely in time and space, even visiting Peladon again. He found her journalistic skills useful in their travels, but he often had to save her.
The Third Doctor regenerated after being exposed to large amounts of radiation during his efforts to stop the Eight Legs of Metebelis III.
