Having been separated from their fleet, a lone Dominator spaceship arrives on the peaceful planet Dulkis.
Navigator Rago and his second-in-command, Probationer Toba, are accompanied by their robotic servants, the Quarks, who round up slave workers from the planet’s pacifist inhabitants.
Needing to refuel their vessel, the Dominators begin drilling into the surface of the planet, where they plan to fire their rockets into the molten core of Dulkis, then drop an atomic-powered seed pod which would turn the whole planet radioactive.
When the Second Doctor, Jamie and Zoe arrive, they are unable to persuade the Dulcian leaders, who believe violence is never an option, to retaliate.
Assisted by the rebellious native, Cully, the Doctor steals the seed pod from the drilling site, smuggling it on-board the departing Dominator ship. As it takes off, the pod explodes, destroying the alien invaders with their own weapon.