At the inception of the series the Doctor was a mysterious character and little was known about him except that he had a granddaughter, Susan Foreman, and that they were from another time and another world. He had a
on the inside than on the outside. He and Susan were in exile as well, for unspecified reasons. 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 began with schoolteachers Ian Chesterton and Barbara Wright investigating the mystery of Susan, a student who appeared to possess scientific and historical knowledge far beyond her years. Discovering the TARDIS in a scrapyard, they were involuntarily taken by the Doctor on a journey to the year 100,000 BC, and spent two years adventuring through time and space with the Doctor.
It was during 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 someday. The TARDIS crew were soon joined by Vicki, whom they rescued from the planet Dido.
At the conclusion of a chase through time, Ian and Barbara used a Dalek time machine to go home (The Chase), and their place in the TARDIS was taken by a space pilot named Steven Taylor. Together, they met another member of the Doctor's race for the first time in the form of the Meddling Monk and had an adventure in Galaxy 4. During the siege of Troy, Vicki decided to leave the TARDIS to stay with Troilus. The Doctor and Steven were next briefly joined by Katarina and Sara Kingdom, but both were killed during the events of The Daleks' Master Plan.
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. One of the First Doctor's most deadly foes was the Celestial Toymaker, who forced him and his companions to play deadly games. 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), and Dodo deciding to remain on Earth in 1966. The Doctor was then joined by Polly and 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 then-companions Ben and Polly, his cells renewed themselves for the first time, giving him a completely new physical appearance and character – the Second Doctor.