As the TARDIS heads to the paradise planet Florana where the Doctor and Sarah go for a holiday, the TARDIS goes off-course and arrives on the barren planet Exxilon, where the TARDIS is loosing power. The Doctor and Sarah finds a group of humans and Daleks are searching for Parrinium, a mineral which is the only antidote to a space plague. Both parties encounter the native people and trouble ensues. In an attempt to find what caused the power-loss, the Doctor, chased by the Daleks, enter the Exxilon's lost city to be able to stop the power draining phenomenon.

The Doctor and Sarah meet up with Bellal, who helps them escape from the Daleks and explains the origin of the City.