Barbara Wright and Ian Chesterton, two humble teachers during 1963, are surprised by a bright student named Susan Foreman. Feeling inquisitive of her upbringing, they seek out her residence to learn who nurtured such a genius. There, they discover a junkyard inhabited by her grandfather, simply known as "the Doctor", and he doesn't want them lurking about.

When the teachers refuse to leave, they discover that an ordinary police box is actually bigger on the inside. The Doctor decides they know too much about his and Susan's otherworldly origins and takes them on a journey across space and time in his TARDIS, the place he and Susan now call home.