The TARDIS suffers a power drain and is forced down on the planet Exxilon.
The Doctor and Sarah are en route to Florana for a vacation when the Doctor’s TARDIS suffers a series of power failures. An unknown force somewhere on the nearby planet Exxilon is causing the energy drain. They barely land on the arid planet and go to find the source. The Doctor and Sarah exit the TARDIS, but Sarah goes back to change clothes (she’s wearing a swimsuit). The Doctor, exploring, is pursued and captured by the primitive, xenophobic natives.
Sarah searches for the Doctor but, after finding his lamp covered in blood, is chased by the natives. She escapes to an enormous City that pulses with energy, but is captured by the native Exxilons. They consider her presence near their sacred city an abomination and prepare her for sacrifice.
The Doctor, meanwhile, encounters a Marine Space Corps expedition whose ship has also crash landed because of the power drain. The team – Lieutenant Dan Galloway, Lieutenant Peter Hamilton, Captain Richard Railton, and civilian geologist Jill Tarrant – are in search of the rare mineral parrinium, found only on Exxilon. It is needed desperately to cure a galactic plague. Along with Commander Stewart (badly wounded after a previous Exxilon attack), they are the only survivors of their expedition. Another ship lands nearby; the Marines initially mistake it for a rescue vessel, but a squad of Daleks emerge, guns at the ready.