After contracting radiation poisoning from the crystals of the planet Metebelis 3, the Third Doctor makes his way back to UNIT headquarters, where the Time Lord K'Anpo Rimpoche aids him in regenerating (Planet of the Spiders).
In his new incarnation, the Doctor is eager to leave Earth in favour of exploration, thus drawing back from continuous involvement with UNIT (with which he had worked closely as the Third Doctor). He has also grown tired of working for the Time Lords. Despite attempts to avoid them altogether, the Time Lords continue to send him on occasional missions, including an attempt to prevent the creation of the Daleks (Genesis of the Daleks), during which he also meets a new adversary, Davros. The Doctor travels with journalist Sarah Jane Smith, whom he had befriended prior to his regeneration, and, for a time, with UNIT Surgeon-Lieutenant Harry Sullivan.
After a battle with Zygons in Scotland, Harry (having just spent an entire season with the Doctor as they tried to get back to the TARDIS) decided that taking the train was safer than the TARDIS, which the Doctor and Sarah chose to try to make an appointment in London. Instead they ended up on the planet Zeta Minor (Planet of Evil), located at the far edge of the known universe. From this point on the Doctor and Sarah travelled alone.
The Doctor's companionship with Sarah Jane came to an end when he received a telepathic summons to Gallifrey, as humans were not then allowed on the planet. The summons turns out to be part of a trap set by his enemy the Master. The renegade Time Lord has used up all his regenerations and has degenerated into little more than a withered skeletal husk. The Doctor is framed for the assassination of the President of the High Council of Time Lords and put on trial. In order to avoid execution (by vaporisation), the Doctor invokes an obscure law and declares himself a candidate for the office, giving himself the time he needs to prove his innocence and expose the real culprit. This ultimately results in a climactic battle with the Master (The Deadly Assassin).
The Doctor is seen to travel alone for the first time since season 1, returning to a planet he had visited centuries before. During his previous visit, he had accidentally imprinted his own mind on a human colony ship's powerful computer, Xoanon, leaving it with multiple personalities. On his second visit the Doctor is now remembered as an evil god by the descendants of the colonists, some of whom had become a warrior tribe called the Sevateem. After the Doctor cures the computer, one of the Sevateem,Leela, joins him on his travels (The Face of Evil). The Doctor brings the intelligent but uneducated Leela to many locales in human history, teaching her about science and her own species' past. In Victorian London, the pair encounters the magician Li Hsien Chang and his master, the self-styled Weng-Chiang (The Talons of Weng-Chiang). Weng-Chiang is revealed to be a time-jumping criminal from the Earth's distant future.
Later, the Doctor and Leela visit the Bi-Al Foundation medical centre, where they acquire the robot dog K-9 (The Invisible Enemy). While K-9 is malfunctioning, a time distortion leads the TARDIS back to contemporary rural England. While investigating the distortion, he and Leela are confronted by an ancient being that feeds on death from Time Lord history, called the Fendahl (Image of the Fendahl). Eventually, the Doctor returns to Gallifrey and declares himself Lord President, based on the election held during his previous visit. This is in fact a ploy to reveal and defeat a Sontaran invasion plan. In the aftermath Leela and K-9 decide to remain on Gallifrey. The Doctor comforts himself by producing K-9 Mark II (The Invasion of Time).
Shortly afterward, the powerful White Guardian assigns the Doctor the task of finding the six segments of the Key to Time, sending a young Time Lady named Romana to assist him. The two Gallifreyans travel to a variety of planets, encountering strange and unusual allies and enemies, gathering the six segments and defeat the equally powerful Black Guardian, who sought the Key for himself. After the conclusion of the quest, Romana regenerates into a new form (Destiny of the Daleks).
In an effort to evade the Black Guardian, the Doctor installs a "Randomizer" in the TARDIS so that not even the Black Guardian can anticipate where they go. Ironically, the first place the Randomizer sends them is the home planet of the Daleks, Skaro (Destiny of the Daleks). Perhaps because of this, the Doctor begins frequently overriding the machine, first travelling to Paris for a holiday, only to get caught up in an alien scheme to steal the Mona Lisa (The City of Death). He eventually discards the device altogether, remarking that he's fed up with not knowing where he's going.
Shortly after this, the Fourth Doctor and Romana are projected outside the known universe and into a universe of negative coordinates, known as Exo-Space. The TARDIS lands on a planet called Alzerius (Full Circle), where they are joined by a young prodigy named Adric. It's in E-Space that the Doctor destroys the last of a race of giant Vampires who had once threatened all life in his universe. Eventually, the Doctor and his two companions find themselves in a white void with no coordinates, a sort of membrane between the two universes. A way out soon forms, but Romana and K-9 chose to remain behind to help free a race of enslaved creatures in E-Space (Warriors' Gate).
The Doctor and Adric have only just made it back when they're asked to help the people of Traken from a creature known as "Melkur." On Traken, Adric and the Doctor are introduced to the aristocratic Nyssa of Traken. Both Nyssa and her father, Tremas, assist the Doctor in stopping Melkur, who is in fact revealed to be another TARDIS that is controlled by the Master. The Master is narrowly defeated, but managed to take over Tremas' body, thus giving himself a new incarnation.
The Doctor decides to travel to Earth to scan a real Police Box as part of a plan to repair the "Chameleon Circuit", the shape-changing mechanism in the TARDIS. However, the Doctor soon spots a mysterious ghostly figure looking at him in the distance. He eventually confronts the figure, who warns him of future dangers.
As the Doctor prepares to travel to the planet Logopolis to get the Chameleon Circuit fixed, Tegan Jovanka appears in the console room (having previously gotten lost in the corridors of the TARDIS). The conduit between E-Space and our own universe is revealed to be a Charged Vacuum Emboitment (CVE) created by the mathematicians of Logopolis as part of a system to allow the Universe to continue on past its point of heat death. Nyssa shows up, explaining that she was brought to Logopolis by the same figure that the Doctor encountered. Logopolis soon falls under the Master's control, but the stasis field he is generating ends up releasing Entropy and eroding matter throughout the universe, threatening to destroy the entire universe!
The Master agrees to help the Doctor stop the spread of Entropy by adapting the Pharos Project radio telescope on Earth so that they are able to reopen the CVEs. However, when the Master tries to take control of it, the Doctor runs out under the upturned radio dish to sever the cable linking the Master to the CVEs. The Master makes the dish start rotating so that the Doctor will fall to his death. Before he falls, he manages to tear out the cable, only to leave his companions watching as he clings to the cable. As his grip begins to slip, he sees visions of all the enemies he's faced over the years, then falls. Adric, Nyssa, and Tegan gather around the mortally wounded Doctor and call out his name. The Doctor begins seeing visions of all his companions and even the Brigadier calling his name.
He then looks up at the three of them and utters his last words: "It's the end-- but the moment has been prepared for..." He then motions to the white-clad figure of the Watcher, who begins approaching the Doctor. The Watcher, a manifestation of the Doctor's future incarnation, merges with the Doctor and triggers his regeneration. "So he was the Doctor all the time?" remarks Nyssa, as the three watch him transform into the Fifth Doctor.
The Fourth Doctor appears once more in the 20th anniversary special The Five Doctors. A renegade Time Lord attempts to pull the first five incarnations of the Doctor out of time, inadvertently trapping the Fourth Doctor (and Romana) in a "time eddy" from which they are later freed. The Fourth Doctor also had a small cameo at the beginning of Dimensions in Time, warning his Third, Fifth, Sixth and Seventh incarnation to watch out for The Rani. Brief holographic clips of the Fourth Doctor appear in "The Next Doctor" and "The Eleventh Hour"
To an extent, the Fourth Doctor is one of the most unpredictable in terms of his emotional depth, slightly more distant and alien than his previous incarnations.
Despite his obvious moments of whimsical charm, offbeat humour, permeated by his manic grin, the Fourth Doctor is more aloof and somber than his previous incarnations. He could become intensely brooding, serious and even callous. He also displays a darker edge to his personality and in The Invasion of Time he seems to cruelly taunt and play with the Time Lords, after his emergency inauguration as President. He also has a strong moral code, such as when he faces the dilemma of whether to destroy the Daleks in (Genesis of the Daleks) stating that if he did, he would be no better than the Daleks himself. He is truly appalled at the actions of the Pirate Captain in The Pirate Planet and refuses to listen to Professor Tryst's attempts to justify drug-running in order to fund his scientific work (Nightmare of Eden), simply telling him to go away.
At the same time, he is capable of moments of genuine warmth. In The Ark in Space, he salutes the human race's indomitability and latter stories establish that Earth is his favourite planet (The Ribos Operation). He is the first Doctor to refer to his companions as his best friends.
To his companions, especially Sarah Jane Smith, he was protective and somewhat of a father figure. In stories, such as Pyramids of Mars, he is concerned that he is approaching middle age with almost melancholic weariness (something which becomes the main focus of his personality in his final season). He often contemplates his outsider status to both humanity and his Gallifreyan heritage, as he seems more inclined toward a solitary existence (The Deadly Assassin). In contrast to this "outsider existence" he emphasises that he found mankind to be his "favourite species" as if he was scientifically studying it. He could also be furious with those he saw as stupid, frivolous, misguided or just plain evil. When taking charge, he could be considered authoritative to the point of controlling and egocentric. He generally maintained his distance from the Time Lords, remarking in The Pyramids of Mars that, while being from Gallifrey, he doesn't consider himself a Time Lord. He clearly resents that even after they had lifted his exile, they continue to beckon the Doctor whenever they deemed it necessary (Genesis of the Daleks).
Although like all his other incarnations, he preferred brain over brawn. He is a capable swordsman (The Androids of Tara) and fighter when needs dictate, following on from the martial expertise of his immediate predecessor. He improvises non-lethal weaponry when necessary (Genesis of the Daleks), but was also not averse to more lethal weaponry as a necessity against both sentient and non-sentient beings, like the matter-destroying DeMat Gun (The Invasion of Time) or contemporary firearms (Image of the Fendahl and The Talons of Weng-Chiang).
One of the Doctor's most significant relationships occurs during his fourth incarnation and is explored further in his tenth incarnation. His friendship with Sarah Jane Smith is implied to be deeper than the relationships he shared with other companions to that point (as alluded to in the Tenth Doctor episode "School Reunion"). She is consequently still profoundly affected by their separation many years later in her personal timeline and he admits to loving her.
The Fourth Doctor also takes a liking to jelly babies.