The Doctor, Sarah Jane and Harry teleport to Earth to ensure the planet is safe for the survivors on board Nerva Beacon to return and reinhabit their world. A Sontaran named Styre has captured a group of humans and is conducting experiments on them to discover the human body's weaknesses, as part of the Sontaran's goal for domination of the galaxy.

The Doctor learns that Styre has been experimenting on the Galsec crewmembers to pave the way for a Sontaran invasion of Earth and decides the only way to halt his plans is to challenge him to a duel.