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The Third Doctor was exiled by the Time Lords on Earth for a significant period of his life. The technological limits of the time period often made him frustrated and bitter. However, he retained his endearing compassion toward human companions.
Authoritative, yet rebellious, he was quick to demand action just as much as he felt compelled to take action himself. If unsatisfied with someone's handling of a situation, this incarnation would step in forcefully and take charge, especially if they seemed aggressive and undiplomatic.
At times, he could become argumentative and needed to be defused in order to establish fluid communication with his associates, but when cheerful, could be quite humble and sociable, with a few humorous anecdotes thrown in for good measure.
His foremost archenemy was the Master, a result of the Time Lord's fiendish desire to destroy the life on the planet of his exile, but almost in a recreational sense. Even though he was villainous, the Doctor treated their relationship as a friendly rivalry, often thrilled to face him in a battle of wits, while the Master reflected an equally casual approach back at him. However, he was also determined to rehabilitate the Master, who he once knew as a benevolent familiar before he fell astray. Though the Master sometimes teamed up with the Doctor to deal with foes who threatened them, he always bent to dark and malicious desires, which kept their mutual antagonism alive.
This Doctor worked for UNIT as their unpaid scientific advisor. He remained in their service even after his exile was ended. Unlike his earlier incarnations, he was eager to engage others physically. He also cut a more dashing figure.
In the field, the Doctor was aided by Sergeant John Benton, Captain Michael Yates and most notably Brigadier Alastair Gordon Lethbridge-Stewart. The Doctor did not get along well with the Brig at first, but they ultimately developed an easy mutual trust and became good friends.
He was initially assisted in the laboratory by Dr. Elizabeth Shaw. 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.
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, however, 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 and succumbing to ten years of radiation poisoning while lost in the time vortex.
Notes for Doctor Who