One of their students, Susan Foreman, the granddaughter of the Doctor, shows an unusually advanced knowledge of science and history. Attempting to solve the mystery of this "unearthly child", Ian and Barbara follow Susan back 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.