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 in combating the threats they encountered. There is some disagreement about when the Third Doctor's UNIT stories were set, with some evidence that they were contemporary stories set at the same time they were broadcast (the early 70s) and some evidence that they were set in the near future. According to the production team, there was an intention to set the stories in the near future, but the writers did not always remember this and set the stories in the present.
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. As well as the Brigadier, he developed friendships with other regular UNIT colleagues including Sergeant Benton and Captain Mike Yates. When meteorites 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. The Autons were to be one of the Doctor's recurring foes. At the conclusion of this adventure, the Doctor became UNIT's scientific advisor. 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.
After meeting Jo, the Third Doctor encountered his greatest nemesis — the Master. 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. The Doctor's exile continued until it was lifted by the Time Lords after he helped save them from destruction at the hands of Omega. The Third Doctor, free to roam space and time again, soon ran into the Master and an even older enemy — the Daleks. Although the Master was a criminal genius, the Doctor was always able to outwit him in all his schemes. Whilst facing the ecological destruction wrought by Global Chemicals and the super computer BOSS, Jo met and fell in love with Dr. Clifford Jones. Marrying Jones and following him to the Amazon on an expedition, Jo left a saddened Doctor.
The fiercely independent investigative journalist Sarah Jane Smith became the Doctor's new companion after stowing away in his TARDIS. The Third Doctor's final adventures saw them defeating the Sontarans in medieval England and the Daleks on the planet Exxilon. The Third Doctor contracted radiation poisoning on the planet Metebelis 3, during the events of Planet of the Spiders. When the TARDIS brought him back to UNIT headquarters, he collapsed, regenerating into the Fourth Doctor.
The Third Doctor was a suave, authoritative man of action, who not only practiced Venusian Aikido (or Karate), but who enjoyed working on gadgets and riding all manner of vehicles, such as the Whomobile and his pride and joy, the canary-yellow vintage roadster nicknamed "Bessie" which featured such modifications as a remote control, dramatically increased speed capabilities and even inertial dampeners.
While this incarnation had spent most of his time exiled on Earth, where he grudgingly worked as UNIT's scientific advisor, he would occasionally be sent on covert missions by the Time Lords, where he would often act as a reluctant mediator. Even though he developed a fondness for Earthlings with whom he worked (such as Liz Shaw and Jo Grant), he would jump at any chance to return to the stars with the enthusiasm of a far younger man than himself (as can be seen in his frivolous attitude in The Mutants). If this Doctor had a somewhat patrician and authoritarian air, he was just as quick to criticise authority too—having little patience with self-inflated bureaucrats, parochially-narrow ministers, knee-jerk militarists or red tape in general. His courageousness could easily turn to waspish indignation. It is thus no surprise that another catchphrase of his, perhaps less well-known but more common overall, was "Now listen to me".
Despite his arrogance, 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. In fact, even when his much resented exile was lifted, the highly moral and dashing Third Doctor continued to help UNIT protect the Earth from all manner of alien threats.
In general, this incarnation of the Doctor was more physically daring than the previous two, and was the first to attack an enemy physically if cornered (both of his previous incarnations would nearly always attempt to dodge, flee or attempt to persuade hostiles to stop their attack rather than directly defend themselves). 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. He would only use his fighting skills if he had no alternative, and even then generally disarmed his opponents rather than knock them unconscious. Indeed, his martial prowess was such that a single, sudden strike was usually enough to halt whatever threatened him, and at one point he reminded Captain Yates (physically as well as verbally) that he would have a difficult time removing him from somewhere, when he did not want to be removed (The Mind of Evil).
Perhaps due to his time spent on Earth, or maybe just as a function of his pacifistic and authoritarian tendencies, the Third Doctor was a skilled diplomat (keeping talks going in The Curse of Peladon, for example) and linguist, as well as having an odd knack for disguises - all of this, combined with his formidable galactic experience, often allowed the Third Doctor to play a central role in the events he found himself in.
Occasionally camp but always charismatic, this Doctor had a personal manner of dress which is the most ornate of his various incarnations, favouring frilled shirts, velvet smoking or dinner jackets in blue, orange, green, burgundy or black, evening trousers, formal boots and opera and hunting cloaks for his regular outfit, with variations and accessories including bow ties, cravats and leather gloves, earning him the nickname "the Dandy Doctor". In The Three Doctors, the First Doctor, commenting on the Third and Second Doctor respectively, disparagingly referred to them as "a dandy and a clown".
The Doctor Who Site