The Third Doctor was the third incarnation of the Time Lord known as the Doctor. He was exiled by the Time Lords on Earth for much of his life. The technological limits of the time period often made him frustrated and bitter. This Doctor worked for the United Nations Intelligence Taskforce as their unpaid scientific advisor. He remained in their service long after his exile was ended. Unlike his earlier incarnations, he was eager to engage others physically. The Third Doctor regenerated after being exposed to large amounts of radiation during his efforts to stop the Eight Legs of Metebelis III.