The Doctor builds a machine designed to predict the future. It shows the Brigadier murdering him and Jo in cold blood. Unable to tell where or when this event is destined to occur, the Doctor and Jo decide that they must stay apart.

Jo is sent on a top secret mission to the war torn Arab nation of Kebiria. But upon arrival, she is immediately arrested and consigned to a brutal political prison. The Kebirians have something to hide: deep in the North African desert, an alien infestation is rapidly growing. And the Doctor and UNIT soon discover that unless stopped, the alien presence will spread to overrun the entire world.