The Doctor is off on another mission for the Time Lords (who presumably figure that giving him busywork will keep him out of trouble) - he has a sealed message to deliver to recipients unknown aboard a space station orbiting the planet Solos in the 30th century. Solos is about to gain independence from Earth's Empire, but the Marshal in command is determined to prevent this and is working with scientist Jaeger to convert Solos's atmosphere into an Earth-like one which will kill the Solonians.