Ian Chesterton is a science teacher at the Coal Hill School and works with Barbara Wright, a history teacher. One of their students, Susan Foreman, the granddaughter of the Doctor, shows unusually advanced knowledge of science and history. Attempting to solve the mystery of this "unearthly child," Ian and Barbara follow Susan back home to a 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 a time machine known as the TARDIS, and are whisked away on an adventure in time and space with the Doctor and Susan.
Ian provides the series with an action-oriented figure, able to perform the physical tasks that the elderly Doctor can not. His concern, above all, is for the safety of the TARDIS crew, and in the early stories he often takes issue with the Doctor's habit of placing the group in harm's way just to satisfy his own curiosity. The chemistry between Barbara and himself is also evident, although the nature of their relationship is never made explicit in the television series.
Ian shows a surprising breadth of skills throughout his tenure with the Doctor. He manages to create fire, rides a horse, knows how to fight with swords, and is knowledgeable about pressure points that can paralyze an opponent. He is also fiercely protective of Barbara, going on a lone mission to rescue her from Saracens in The Crusade. In that story, he is also knighted by King Richard I of England as "Sir Ian of Jaffa," although presumably he would be unable to use that title in his own time. After many travels, Ian and Barbara eventually use a Dalek time machine to get home, albeit two years after their disappearance and presumably with much explaining to do to their friends and families.