School teachers Ian and Barbara follow their student, Susan Foreman, back to her home which is just an old junk yard. There they meet an old man and suspect him of abducting Susan.

With 1963 London left behind, the teachers are amazed to find themselves on prehistoric Earth and are taken captive by a tribe of cavemen who are searching for the secret of fire.

The time travellers manage to escape the tribe but their path to the TARDIS through the forest holds dangers of its own. Meanwhile, Kal attempts to take advantage of events.

Kal has been banished from the tribe and Ian is making fire for Za in the hope that he will be grateful and will allow them to return to the TARDIS.