Written by Norman Ashby
Directed by Morris Barry
The TARDIS materializes on the planet Dulkis, currently under threat from two alien Dominators, Rago and his subordinate Toba, who have landed their spaceship on a remote island.
Aided by their robotic servants the Quarks, as well as slave workers drawn from the native Dulcian population, the Dominators set about drilling holes into the crust through which they plan to fire rockets into the planet's molten core. Their intention is then to drop an atomic seed capsule into the resulting eruption, turning Dulkis into a radioactive mass; fuel for the Dominators' space fleet.
The Dulcian Councilors, being pacifists, refuse to retaliate. Cully, the rebellious son of their leader Senex, has already joined forces with the TARDIS crew. The Doctor eventually defeats the Dominators by intercepting the seed capsule as it is dropped and surreptitiously placing it on board their ship, which is then destroyed shortly after take off.
Dulkis suffers only a minor volcanic eruption, as a result of the rockets fired into its magma. The Doctor and his friends narrowly make it into the TARDIS as advancing lava flows towards them.