The Fourth Doctor's regeneration into the Fifth was a problematic one, and nearly failed, with the Doctor briefly taking on personality aspects from his four previous incarnations. After recovering in the fictional city Castrovalva (actually an elaborate trap created by his arch-enemy The Master), he continued his travels with Tegan Jovanka, Nyssa and Adric. Initially his travels centred on getting Tegan back to Heathrow Airport in time for her first day as an airhostess, but the TARDIS repeatedly missed this destination and Tegan eventually decided to stay in the TARDIS. After trips to the future and the past encountering villains such as Monarch and the Mara, the Fifth Doctor was confronted with tragedy when Adric died trying to stop a space freighter from crashing into prehistoric Earth (Earthshock).
Following Adric's death, the TARDIS accidentally arrived at Heathrow airport (Time-Flight). Here the Doctor and Nyssa left Tegan assuming she would want to stay (when in fact she did not any more). The Doctor and Nyssa then travelled together for an unspecified amount of time before the renegade Time Lord Omega, attempting to return to our universe, temporally bonded himself to the Doctor (Arc of Infinity). Faced with this threat, the Time Lords were forced to attempt executing the Doctor, but he eventually tracked Omega to Amsterdam where he defeated him and re-encountered Tegan (who having now lost her job, had no second thoughts about rejoining the TARDIS crew).
When the Doctor met a new companion, an alien boy stranded on Earth by the name of Vislor Turlough, he did not know that Turlough had been commissioned by the Black Guardian to kill him. Soon after, Nyssa left to help cure Lazar's Disease on the space station Terminus. After meeting the entities known as Eternals racing in yacht-like spacecraft for the prize of "Enlightenment", Turlough broke free from the Black Guardian's influence, and continued to travel with the Doctor and Tegan. Landing in the reign of King John, the crew again encountered The Master, who was using a shape-shifting robot Kamelion to impersonate the King. However, the Doctor helped Kamelion to regain his free will and the robot joined him in his travels (although he rarely left the TARDIS). The Doctor met three of his previous incarnations when they were summoned to the Death Zone on Gallifrey by president Borusa, who was attempting to gain Rassilon's secret of immortality.
After further adventures in which the Doctor re-encountered old foes, including the Silurians and the Sea Devils, both Tegan and Turlough left the TARDIS. Tegan would find the death and violence they encountered on their travels too much to bear (Resurrection of the Daleks), and Turlough returned to his home planet of Trion in the company of his younger brother, as well as other exiles of Trion, from the planet Sarn. The Doctor was eventually forced to destroy Kamelion, when the Master used his mental connection to the robot to regain control of him, a process the robot realised was irreversible (Planet of Fire).
Ultimately, the Fifth Doctor and his last companion Peri Brown were exposed to the drug spectrox in its deadly toxic raw form on Androzani Minor. With only one dose of the antidote available, he nobly sacrificed his own existence to save Peri, expressing doubt for the first time that regeneration might be possible this time, then regenerating into the Sixth Doctor.
A sketch of the Fifth Doctor is seen in John Smith's book in the new series episode "Human Nature" (2007). Visions of the Fifth Doctor (alongside other past Doctors) appear in "The Next Doctor" (2008) and "The Eleventh Hour" (2010).
Somewhere in his life he crashed his TARDIS into the TARDIS of the Tenth Doctor and consequently nearly opened a "Belgium sized" black hole because of the paradox caused, which the Tenth Doctor also uses to explain the notably aged appearance of his former self. However the Tenth Doctor, remembering the event, knew how to stop it because he recalled watching himself correct the mistake when he was the Fifth Doctor. ("Time Crash")
The Fifth Doctor was far more vulnerable, sensitive, and reserved than his previous incarnations and often reacted to situations rather than initiating them. Frequently, he made decisions by flipping a coin. Unlike his more authoritative predecessors, he treated his young companions as parts of a team, and often willingly participated in situations under the leadership of someone else who had the strong command presence that he apparently lacked. However, the Fifth Doctor's boyish appearance, nervous energy, and charm all hid the fact that he was a Time Lord of great age, compassion, and experience. He could decipher the ingredients of a drink by smell alone and rosemary made him sneeze.
This Doctor greatly abhorred violence and often hesitated about taking matters into his own hands. For the first time, indecision weighed seriously on the character, and it had its consequences – yet this Doctor was also one of the most courageous of his incarnations. He was willing to take chances with companions like Turlough and Kamelion, who were originally threats, even as he pretended to be unaware of it at first in order to grant his companion the opportunity to do the right thing. He was also willing to make enormous personal sacrifices (Mawdryn Undead) simply to keep his word and liberate others from suffering. In an episode of Doctor Who Confidential, Steven Moffat said that "this Doctor takes the emphasis off the eccentricities and turns it into a pained heroism of a man who is so much better than the universe he is trying to save but cannot bear to let it stand."
The Fifth Doctor was the last to use the sonic screwdriver in the original series, having been destroyed during The Visitation. The Seventh Doctor and subsequent Doctors were later seen using versions of the tool in the television movie and the revived series. In "Time Crash", the Fifth Doctor declined to borrow the Tenth Doctor's sonic screwdriver.