Jo (Katy Manning) and the Doctor (Jon Pertwee) have been sent on a mission by the Time Lords to deliver a package to a recipient on the planet Solos. Arriving on a nearby space station, the Doctor becomes involved in the planet's struggle to gain its independence from Earth's colonial powers. But the current colonial leader, a man known only as the Marshal, is not keen to give up control, and is planning to bomb the inhabitants out of existence.