Fearful of a Thal attack, the Daleks force Susan to trick the Thals. Realising that the Daleks are planning an ambush, the four travellers try to escape and warn the Thals.
Pretending to be a Dalek, Ian finds that that he is trapped inside the casing. The Doctor, Susan and Barbara have no choice but to abandon Ian as the real Daleks arrive and attack.
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.
What is the connection between the strange behavior and killing of a cashier separated from her husband and the attempted murder of her former employer, a wealthy shipping magnate?
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.
