The Doctor is a mysterious character and little is known about him except that he has a granddaughter, Susan Foreman, and that they come from another time and place. He has a ship that travels through time and space, the TARDIS, which is currently disguised as a police box (Susan notes that it used to be able to change to blend in with its surroundings), and is bigger on the inside. The Doctor describes himself and Susan as "exiles" without specifying why or even whether their exile is self-imposed. It would not be until the last adventure of the Doctor's second incarnation that the name of the Doctor's people (the Time Lords) would be revealed, and the third before the name of his home planet (Gallifrey) was first spoken.
The series' first episode opens with a pair of schoolteachers in contemporary (1963) London, Ian Chesterton and Barbara Wright, investigating the mystery of Susan, a student who seems confused and even frustrated at how what she is learning in history and especially mathematics seems to be wrong. Following Susan to her supposed home, they discover the TARDIS in a junkyard, surprising both Susan and her grandfather the Doctor, who launches the ship in response to the discovery. Ian and Barbara are involuntarily taken on a journey back to the year 100,000 BC and spend two years adventuring through time and space with the Doctor, who at this point in the series has no control over the navigation systems of the TARDIS. It was as this incarnation that the Doctor first met the Daleks and the Cybermen, races that would become his most implacable foes. The TARDIS crew also observed many historical events such as the Reign of Terror in revolutionary France, meeting Marco Polo in China and The Aztecs in Mexico. When Susan fell in love with the human resistance fighter David Campbell, the Doctor left her behind to allow her to build a life for herself on 22nd century Earth (The Dalek Invasion of Earth), although he promised to return some day. The Doctor, Ian and Barbara were then joined by Vicki, whom they saved in The Rescue from the planet Dido. At the conclusion of a chase through time by Daleks, Ian and Barbara used the Dalek time machine to go home (The Chase), and their place in the TARDIS was taken by a future space pilot named Steven Taylor, who had been captured by the robot Mechanoids but escaped due to the Dalek attack. The Doctor met another member of the Doctor's race for the first time in the form of theMeddling Monk and had an adventure in Galaxy 4. During the siege of Troy, Vicki decided to leave the TARDIS to stay with Troilus, adopting the name Cressida. The Doctor and Steven were next briefly joined by Trojan slave Katarina and a security agent from 4000 AD Sara Kingdom, but both were killed during the events of The Daleks' Master Plan, where the Daleks plotted to invade Earth's Solar System in 4000 AD, before being destroyed when the Doctor activated their weapon the Time Destructor. Due to this he was aged significantly; the effects on Sara were enough to reduce her to dust. After narrowly missing the Massacre of St Bartholomew's Eve, the Doctor and Steven took on board a young girl named Dodo Chaplet. Dodo brought a cold virus to the far future, which nearly annihilated the humans and Monoids travelling on The Ark. It was cured and when the TARDIS arrived on the Ark 700 years later the TARDIS crew helped the humans reach their destination, the Monoids having taken over the Ark. One of the First Doctor's most deadly foes was the Celestial Toymaker, who forced him and his companions to play deadly games and briefly made the Doctor invisible and mute. Eventually, the Doctor managed to win the Trilogic Game allowing them all to escape the Toymaker's domain. Eventually, Steven and Dodo left the Doctor as well, Steven remaining on an alien planet as a mediator (The Savages) between two races, and Dodo deciding to remain on Earth in 1966 to recover from being hypnotised by the supercomputer WOTAN. The Doctor was then joined by upper-class secretary Polly and sailor Ben Jackson who would witness his first regeneration. The toll of years put strain on the Doctor's elderly frame. After defeating the Cybermen at the Antarctic Snowcap Station (The Tenth Planet), the Doctor collapsed inside the TARDIS, and before the astonished eyes of his companions Ben and Polly, regenerated for the first time, getting him a new physical appearance and character – the Second Doctor.
From the beginning, the First Doctor was a mysterious figure. He appeared to be a frail old man and yet was possessed of unexpected reserves of strength and will. An earlywriters' guide by script editor David Whitaker describes "Doctor Who" as "frail-looking but wiry and tough as an old turkey".He obviously held tremendous knowledge of scientific matters, and yet was unable to pilot his TARDIS time ship reliably; his granddaughter Susan explained this by saying that her grandfather was "a bit forgetful"; but the TARDIS required expert piloting and guidance by the Doctor. Its systems would often break down, especially the navigational systems. This, combined with the fact the TARDIS was actually designed for six pilots, would explain the difficulty the Doctor encountered in piloting it correctly. He was abrasive, patronising, and cantankerous towards his human travelling companions, yet shared a deep emotional bond with his granddaughter Susan. He also harboured a streak of ruthlessness, being willing to lie—and in one case attempt to kill—to achieve his goals. Initially, he distrusted his first two human companions, Ian Chesterton and Barbara Wright, who were forced on him. Over time, however, as they shared adventures together, he grew closer to them, and the TARDIS crew came to share almost a family bond.
Eventually, the Doctor began to enjoy his travels through time and space, taking people along for the ride and was always reluctant and sad to see them go, even when he knew it was for their own good. The Doctor's personality mellowed around the time of the serial Marco Polo, and he evolved into the more familiar grandfatherly figure that children loved.
It was also during this time that the Doctor first met what would become his most persistent adversaries—the Daleks and the Cybermen. It would be the latter encounter that would see the Doctor first regenerate; stating "This old body of mine's wearing a bit thin," he collapsed at the end of the serial, regenerating inside the TARDIS into the Second Doctor.
William Hartnell described the Doctor as "a wizard", and "a cross between the Wizard of Oz and Father Christmas".Paul McGann, who played a later incarnation of the Doctor, said Hartnell's "seemed like a Victorian, someone kind of stern and paterfamilias about him. Something kind but scary".One quirk of the First Doctor was his tendency to become occasionally tongue-tied and stumble over words.Sometimes this was a deliberate acting choice: William Russell recalls that it was Hartnell's idea for the Doctor to get Ian Chesterton's surname wrong, calling him "Chesserman" or "Chatterton".This character choice also gave the series' producers the ability to use takesin which Hartnell flubbed his lines. Due to the series' tight production schedule, it was rarely possible to reshoot such scenes and dubbing the dialogue was usually not an option. Hartnell suffered from undiagnosed arteriosclerosis, which affected his ability to remember lines, increasingly so as his time on the series progressed.