An incorrigible meddler in the affairs of other species, the Doctor is in self-imposed exile from Gallifrey, the planet of the Time Lords. He prefers the freedom of roaming throughout time and space, even in his somewhat unreliable TARDIS, to the dull, regimented existence of the Time Lords. As masters of time travel, his people have a responsible policy of non-interference in the affairs of other species, a practice that the Doctor is constantly trying to prove is wrong: he believes interference is sometimes warranted, especially to balk the forces of evil that are at work in the universes of matter and antimatter.

On Gallifrey, the Doctor is known as a graduate of the Prydon College of the Academy, and is entitled to wear its scarlet and orange ceremonial robes. Prydonians are considered to be the best students out of the Arcalians, who wear green, and the heliotrope-clad Patrexes.

Even as a crusader of time and space, the Doctor is seldom methodical. His TARDIS is as temperamental as he is, to the extent that in his first and second incarnations he has no real control over its destination. There is seldom any consistent plan to deal with enemies, either, and the Doctor is as likely to get caught up in events over which he has no control as he is to stumble across Daleks or Cybermen. The Doctor is commonly mistaken for ambassadors or badly needed experts; his skills are likely to be put to use before he has a chance to explain -- by which time it is too late to protest. If he interferes willingly, he always acts for the best of intentions.

Time Lords have two hearts, a phenomenal constitution that enables them to withstand environments that would kill a human, and an ability to fall into a self-induced trance (in effect, self-hypnotism) to save them from truly harmful conditions. Most remarkable of all their abilities, however, is that of regeneration. Like a snake shedding an old skin, all Time Lords can regenerate their physical form to compensate for massive tissue damage and to avoid impending death. Such regenerations, however, are not always easy on the Doctor or his companions because his new personality takes time to stabilize. A Time Lord, however, can regenerate only twelve times, limiting his form to thirteen incarnations and ultimately restricting his apparent immortality. The Doctor is currently in his ninth incarnation.

Each incarnation of the Doctor has unique traits and abilities; what the Doctor knows in one incarnation does not necessarily transfer to the next one. Parts of his memory open and close, revealing new talents and obscuring old ones. His physical size and appearance change as well, although a new incarnation is likely to be unhappy about the changes to his physiognomy. The trouble with regeneration, as the Doctor says, is that you never know what you are going to get.