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.
UNIT are at a project run by Professor Stahlman who intends to penetrate Earth's crust. But the drilling has already released something deadly from beneath the Earth's surface.
More ducking and diving as Del and Rodney discover that three tons of lead is not all it appears to be. In fact, it could even be used in case of war.
