The Doctor and Romana arrive on a desolate planet, only to find that they are not the only people interested in the remains of an ancient city.
The Movellans try to discover the purpose of the Daleks. The Doctor has a terrible suspicion.
The Doctor's worst fears are realised, but he and Romana are trapped deep underground and the Daleks are moving in for the kill.
The Doctor and Romana face a terrible quandary. How can they defeat the Daleks without unleashing a greater menace?
