Eight years after the events of Prelude, Seldon has worked out the science of psychohistory and has applied it on a galactic scale.
His notability and fame increase and he is eventually promoted to First Minister to the Emperor.
Prelude to Foundation opens on the planet Trantor, the empire's capital planet, the day after Hari Seldon has given a speech at a conference.
Several parties become aware of the content of his speech (that using mathematical formulas, it may be possible to predict the future course of human history).
Seldon's calculations also show there is a way to limit this interregnum to just one thousand years.
To ensure the more favorable outcome and reduce human misery during the intervening period, Seldon creates a foundation of talented artisans and engineers at the extreme end of the galaxy, ostensibly to preserve and expand on humanity's collective knowledge, and thus become the foundation for a new galactic empire, but actually to place a society in a way shown by his calculations to bring around the desired outcome (the Seldon Plan).
The idea is the one developed in Robots of Dawn, which, in addition to showing the way that the second wave of settlements were to be allowed, illustrates the benefits and shortcomings of the first wave of settlements and their so-called C/Fe (carbon/iron, signifying humans and robots together) culture.
In this same book, the word psychohistory is used to describe the nascent idea of Seldon's work.
Using the laws of mass action, it can predict the future, but only on a large scale.
Seldon foresees the imminent fall of the Galactic Empire, which encompasses the entire Milky Way, and a dark age lasting 30,000 years before a second great empire arises.
The remainder of the stories were published in pairs by Gnome as Foundation and Empire (1952) and Second Foundation (1953), resulting in the "Foundation Trilogy", as the series is still known.
Four years later, Asimov followed up with yet another sequel, Foundation and Earth (1986), which was followed by the prequels Prelude to Foundation (1988) and Forward the Foundation (1993).
Seldon is hounded by the Emperor and various employed thugs who are working surreptitiously, which forces him into exile.