Ian and Barbara join a Thal expedition that will penetrate the Dalek city from the unguarded mountains behind it. Their journey is a dangerous one and not all will survive.
The Doctor and Susan are once again taken prisoner by the Daleks. In the mountains, Ian, Barbara and the Thals must negotiate a vast chasm to reach the city.
The Thals and the time travellers attack the Daleks from all sides as the Daleks start the countdown to release deadly doses of radiation into the atmosphere.
