In PRELUDE TO FOUNDATION, what happed in the many centuries before the events made famous in Asimov's other Foundation novels - hitherto only hinted at - is now revealed.
When the First Foundation was conquered by a force Seldon had not foreseen - the overwhelming power of a single individual, a mutant called the Mule - the second Foundation was forced to reveal its existence and, infinitely worse, a portion of its power.