The Doctor is a Time Lord, an extraterrestrial from the planet Gallifrey, who travels in a time machine called the TARDIS — Time And Relative Dimension (or Dimensions) In Space — that allows him to reach any point in time and space and is dimensionally transcendental (larger on the inside than on the outside). The TARDIS originally had the ability to disguise itself according to its environment, but became "stuck" in the form of a police box after landing in London in 1963 and has remained in that shape ever since due to a malfunctioning chameleon circuit. The Doctor has since given up attempting to repair or replace the circuit as he has grown fond of the police box shape.

For the most part, and usually because the vessel's navigation system is old and unreliable, the Doctor explores the universe at random and uses his extensive knowledge of science and advanced technology to heroically avert the crises that he encounters. The Doctor has, at various times, been accompanied by companions who have chosen to travel with him for a variety of reasons.

Although he physically resembles a human, as a Time Lord his physiology is different. Like other members of his race, he has two hearts, a respiratory bypass system that allows him to go without breathing for periods of time, and on occasion exhibits greater strength and stamina than humans. He has also exhibited a resistance to temporal distortions and a sensitivity to changes in time. He also claimed that a pill (intended to be aspirin) could kill him (The Mind of Evil).

The title screen of the original untransmitted pilot of Doctor Who.

In The War Games, the Second Doctor stated that Time Lords could live forever, "barring accidents." When accidents do occur, they can regenerate into new bodies, giving them extremely long life-spans. The 1996 Doctor Who television movie stated that the Doctor was half-human, a revelation that continues to cause controversy among fans (see below).

The Doctor is considered a renegade by the Time Lords for his penchant of getting "involved" with the universe, in direct violation of official Time Lord policy. However, most of the time his actions are tolerated, especially when he has saved not just Gallifrey, but the universe, several times over. The Time Lords were also partial to sending him on missions when deniability or expendibility was needed. The Doctor's standing in Time Lord society has waxed and waned over the years, from being a hunted man to even being appointed Lord President of the High Council (an office he did not assume for very long and eventually was removed from in his absence). In the end, though, he has always seemed quite content to remain a renegade and an exile. Ultimately, the Doctor found himself, by the time of his ninth incarnation, the last known surviving Time Lord.

The character was first portrayed by William Hartnell in 1963, who played him as an irascible, grandfatherly figure. When Hartnell left the series, the role was taken over by Patrick Troughton in 1966. To date, ten actors have played the Doctor on television, with perhaps the most enduring incarnation being the fourth, played by Tom Baker. David Tennant currently plays the Tenth Doctor beginning his tenure at the end of The Parting of the Ways, which was first aired on 18 June 2005.

When the series began, nothing was known of the Doctor at all, not even his name. In the very first serial, 100,000 BC, two teachers from the Coal Hill School in London, Barbara Wright and Ian Chesterton, became intrigued by one of their students, Susan Foreman, who exhibited high intelligence and unusually advanced knowledge. Trailing her to a junkyard at 76 Totter's Lane, they encountered a strange old man and heard her voice coming from inside what appeared to be a police box. Pushing their way inside, the two found that the exterior was actually camouflage for the dimensionally transcendental interior of the TARDIS. The old man, whom Susan called "Grandfather" but simply termed himself "the Doctor", subsequently whisked them away on an adventure in time and space.

As a time traveller, the Doctor has been present at or directly involved in countless major historical events on the planet Earth and on other worlds — sometimes more than once. In the 2005 series premiere Rose, it is revealed that the Ninth Doctor was instrumental in preventing a family from boarding the Titanic prior to her fateful voyage. In The End of World the Doctor claimed to have been on board and survived the Titanic's sinking to find himself "clinging to an iceberg."

Many historical figures on Earth have also encountered the Doctor. In City of Death it was revealed that the Doctor had met Leonardo da Vinci and William Shakespeare and that the first folio of the latter's Hamlet was transcribed by the Doctor himself. In Timelash, he met a young H. G. Wells, Albert Einstein in Time and the Rani, and Marco Polo was the subject of an adventure of the same name during the first season. Most recently, the Doctor shared an adventure with Charles Dickens (The Unquiet Dead). In the 2006 series, the Doctor will meet Queen Victoria in the episode Tooth and Claw and Madame de Pompadour in The Girl in the Fireplace.