The TARDIS materializes on the Dulkis.  Dominators, an alien race, have landed on the planet and the Quarks have taken slaves from the native population.  They start drilling bore holes and plan to send bombs into the planet's molten core.  Their plan is to turn Dulkis into radioactive fuel for the Dominator space fleet.

The pacifistic Dulcian Councillors refuse to act but the Doctor and his companions are able to defeat the Dominators.  They catch the seed capsule before it can reach the core and put it on the Dominator's ship.  The ship is destroyed soon after it launches.