The Time Lords send the Fourth Doctor and Sarah Jane to the supposedly uninhabited planet Sarto. The difficulty is that the two TARDIS travellers don't know precisely why the Time Lords have done this. They tramp through the jungle terrain to discover a massive hydro-electric plant sitting on a huge, Niagara-like waterfall. As they go in for a closer look, the Doctor twigs. The architecture is unmistakably Dalek. The Daleks have built a massive installation here. This explains why the Time Lords were so concerned about Sarto.