London, 1963. Schoolteachers Ian Chesterton and Barbara Wright are perplexed by the behaviour of one of their pupils, Susan Foreman. Her knowledge of science and history exceeds theirs, yet she seems totally ignorant of many common aspects of everyday life. They follow her to her home address, a junkyard with a police telephone box standing in it, and encounter her grandfather, the enigmatic Doctor. When they force their way past him into the police box, Susan's secret is revealed: she and the Doctor are aliens, and the police box is a time machine, the TARDIS, capable of visiting any point in the universe at any moment in time…