Susan was the first ever on screen companion of the Doctor and she stayed on Earth after the Dalek Invasion in the 22nd century. Susan is the granddaughter of the Time Lord known as the Doctor. Her last name of Foreman is an alias taken from the junkyard, owned by an "I. M. Foreman" at 76 Totter's Lane, where she and the Doctor lived (in the TARDIS) during their time in London in 1963.
The Doctor explains in "An Unearthly Child" that he and Susan are exiles from their own people. Susan adds, "I was born in another time, another world". Susan claims to have coined the name for the TARDIS, the Doctor's time machine, though later episodes seemed to indicate that it was a widely used term among Time Lords. It is not known if Susan is the character's real name or an alias to make her appear more human.The series' treatment of Time Lords' names is slightly inconsistent, with some Time Lords using "titles" such as The Doctor, The Master, The Inquisitor and so on, and some Time Lords using more "normal" names such as Borusa, Flavia, Spandrel or Damon.
Susan's age is given as 15. In The Sensorites, the Doctor, when encountering an unconscious young human woman, remarks that "she's only a few years older than Susan," suggesting that Susan is the age of a normal secondary school student.
The Doctor and Susan had already been traveling for an unspecified amount of time, before they decide to settle in London to make repairs on the TARDIS; Susan states that she and her grandfather have been in London for five months. Susan also alludes to several previous adventures to Ian Chesterton and Barbara Wright in the course of their shared adventures; both on various alien planets and on Earth in various past Time Periods (such as encountering a wrathful King Henry the Eighth). Susan begins to attend the Coal Hill School in Shoreditch, where her advanced knowledge of history and science attract the attention of schoolteachers Barbara and Ian. Attempting to solve the mystery of the "unearthly child," Ian and Barbara follow Susan back to the junkyard, where they hear her voice coming from what appears to be a police box. When they investigate further, they discover that the police box exterior hides the much larger interior of the TARDIS; and are whisked away on an adventure in time and space with the Doctor and Susan, against their will.
Susan continues to travel with the Doctor and her two teachers until the 1964 serial, The Dalek Invasion of Earth. During the events of that story, Susan falls in love with David Campbell, a young freedom fighter in the 22nd century. However, Susan feels that she has to stay with and take care of her grandfather. The Doctor, realizing that Susan is now a grown woman and deserves a future away from him, locks her out of the TARDIS and leaves after a tearful farewell. Carole Ann Ford had expressed a desire to leave the series as she felt the character of Susan was too limiting. Ford reprised the role of Susan on television in the 20th anniversary special The Five Doctors, but no mention of David, or what became of him, was made.