This text describes a science fiction adventure where the Doctor and his companions, Sarah Jane Smith and Harry Sullivan, arrive on a desolate future Earth. They encounter a group of stranded human colonists from the Galsec colony who are being hunted by a sadistic Sontaran warrior named Styr. Through various experiments and torture, Styr is assessing human weaknesses as a prelude to a planned galactic invasion. The Doctor eventually thwarts these plans by challenging Styr to single combat and sabotaging the alien's recharging equipment. By using his scientific ingenuity and tools like the sonic screwdriver, the Doctor defeats the Sontaran, saves the surviving humans, and departs via a matter transmitter.