Within the story, the Third Doctor came into existence as part of a punishment from his own race, the Time Lords, who forced him to regenerate and also disabled his TARDIS.
After the Doctor was found guilty of breaking the Time Lord laws of non-interference and forced to regenerate, he began his third incarnation in exile on 20th century Earth.
The Third Doctor immediately formed a working relationship with the British contingent of UNIT, an international organisation tasked to investigate and defend the Earth against extraterrestrial threats.
It was a partnership initially born out of convenience — the Doctor required facilities to try to repair his TARDIS to break the exile, and UNIT needed his expertise to combat the threats they encountered.
The Doctor also developed a good working relationship with Brigadier Lethbridge-Stewart, whom he had first encountered, in his previous incarnation, as a Colonel in command of troops fighting Yeti and the Cybermen.
When meteors were seen falling to Earth in Essex, the Doctor together with a UNIT scientist named Liz Shaw were to face the Autons for the first time.
After facing Silurians, the so-called Ambassadors of Death and the Inferno project, Liz was replaced as the Doctor's assistant by a feisty but slightly scatter-brained young woman named Jo Grant.
A renegade Time Lord, the Master plagued the Third Doctor with his diabolical schemes, including the summoning of an ancient Dæmon, and unleashing the terrifyingly powerful Kronos, a Chronovore.
Whilst facing the ecological destruction wrought by Global Chemicals and the super computer BOSS, Jo met and fell in love with Dr. Clifford Jones.
His favourite car was a canary-yellow vintage roadster that he nicknamed "Bessie", a machine which featured such modifications as a remote control, dramatically increased speed capabilities and inertial dampers.
[3] Despite his occasional arrogance,[1] the Third Doctor genuinely cared for his companions in a paternal fashion, and even held a thinly veiled but grudging admiration for his nemesis, the Master, and for UNIT's leader, Brigadier Lethbridge-Stewart, with whom he eventually became friends.
This often took the form of quick strikes, with the occasional joint lock or throw — usually enough to get himself and anyone accompanying him out of immediate danger, but usually not to the extent of a brawl, in keeping with the Doctor's non-violent nature.
[4] When asked to attend a Radio Times photo-call in 1969, Jon Pertwee arrived in what he thought was "a suitably eccentric outfit" from his family wardrobe, and the flamboyant image stuck with producer Barry Letts.
[5] Through the first two seasons, he wore a flowing, crimson-lined cape over a black velvet smoking jacket and a ruffled shirt with a variety of neckties such as jabots, bow ties or cravats.
Eventually, after his defeat of the renegade Omega in The Three Doctors, he was granted complete freedom by the Time Lords in gratitude for saving Gallifrey.
The Third Doctor's era introduced adversaries including the Autons, the Master, Omega, the Sontarans, the Silurians and the Sea Devils.
In "The Girl Who Died", the Twelfth Doctor tells Clara Oswald he is "Reversing the polarity of the neutron flow", followed by "I bet that means something.
Clara herself uses the phrase, saying she "reversed the polarity" of a mind-wiping device to prevent the Doctor from erasing her memories of him from her mind ("Hell Bent").
In "It Takes You Away", Yaz Khan suggests that the Thirteenth Doctor reverse the polarity on the sonic screwdriver in order to (successfully) open a locked inter-universe portal.
[13] It features red, black then green flaming hands, then shows Jon Pertwee's face followed by a series of swirling lines to represent the time vortex.