A mysterious power-loss strands the TARDIS on Exxilon, a sinister fog-shrouded alien planet. Forced to brave the dangers of the planet, the Doctor meets the survivors of a beleaguered expedition from Earth searching for a precious mineral that can save the galaxy from a terrible space-plague. Sarah finds a mysterious super-City and becomes a captive of the savage Exxilons, and, worst of all, the Doctor's greatest enemies, the dreaded Daleks, arrive on a secret mission of their own.

What terrifying power makes captives of all who come to the planet? What is the secret of the mysterious deserted City with its great flashing beacon? And what sinister plan has brought the Daleks to Exxilon? The Doctor and Sarah must risk their lives time and again in a desperate attempt to foil the Daleks and save millions of humans from the horrific plague.