At Coal Hill School, teachers Barbara Wright (Jacqueline Hill) and Ian Chesterton (William Russell) become intrigued by the strange behaviour of one of their pupils, Susan Foreman (Carole Ann Ford). Barbara has discovered that the girl's home address, 76 Totter's Lane, is the site of a junk yard. She persuades Ian to accompany her there after school one evening to see where Susan goes.

Ian and Barbara follow Susan through the junk yard gates, but once inside can find no sign of her. Their search is interrupted by the arrival of an old man (William Hartnell) dressed in Edwardian-style clothes. They grow suspicious, believing that he may have imprisoned Susan within a police box they have discovered in the yard, but he refuses their request to unlock it. They are just on the point of fetching a policeman when the doors open of their own accord and Susan calls out. The two teachers push their way past the old man and into the police box.

Inside, they find themselves in a huge, brightly-lit room control room. Susan explains that the old man is her grandfather, the Doctor, and that this is a time machine called TARDIS, standing for Time And Relative Dimension In Space. Ian and Barbara try to persuade her that this she is fantasising, but she maintains that she and her grandfather were born in another time and on another planet. The two teachers decide to leave, but find that the doors are now closed. The Doctor explains that he is unwilling to let them go as they will inevitably tell others what they have discovered. Susan pleads with him to change his mind, but instead he activates the ship's controls and sends it on a journey through time.

The ship arrives in the Stone Age, where its four occupants encounter a tribe whose leader, Za (Derek Newark), is striving to rediscover the lost secret of fire. He is encouraged by the young woman Hur (Alethea Charlton) but mocked by Old Mother (Eileen Way), who believes that fire would bring only trouble. The elders, including Horg (Howard Lang), are growing impatient with Za and beginning to wonder if the newcomer Kal (Jeremy Young) might make a better leader.

Kal seizes the Doctor after seeing him light his pipe and takes him back to the tribe, demanding that he make fire. The Doctor, however, has dropped his matches in the skirmish and is unable to do so. The other travellers are also taken prisoner and all four are then placed in the Cave of Skulls - a cave full of skeletal remains, the skulls of which have been split wide open.

Ian eventually makes fire for Za by rubbing a stick against a stone to create friction, but Za - having killed Kal in a fight and confirmed his leadership of the tribe - insists that they remain prisoners in the Cave. Susan eventually devises a plan to scare the tribespeople by placing skulls on top of burning torches. While Za and his people are distracted by this gruesome sight, the travellers make good their escape. They reach the safety of the ship just before the angry tribe catch up with them, and the Doctor sends it on its way again.

Synopsis from Doctor Who: The First Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos