The ambitious general from the Galactic Imperium, Bel Riose, decides that now is the best time for conquering the Foundation.  General Riose shows some initial successes, and in fact it looks pretty bad for the foundation as they don't seem to be capable of providing an answer to Riose's strategy.  But then the Emperor feels that having an ambitious general is one thing, but an ambitious successful general is a threat, so he simply removes the threat.  And the Foundation is once again saved, as predicted by Hari Seldon, so many centuries ago.  Then however the Foundation learns of a second threat: "The Mule".  A far more dangerous threat, since The Mule is just that what the Psychohistory could not predict, nor do thay have an answer to his presence in the galaxy; The Mule is a mutant.  And nothing seems to be capable of stopping The Mule from adding the Foundation to his own private empire...

Asimov's original idea for this series, the "Psychohistory"-science, is potentially a killer for the stories.  After all, it is capable of predicting everything a large group of normal individuals do, and provide an answer to those actions - or at least an explanation.  Which in essence potentially maes boring stories: you don't know how yet, but in the events will somehow lead to the growth of the Foundation.  That is the weak point of the first story, about general Bel Riose.  And the strong point about the second, with the invisible and unpredictable Mule.

I shouldn't give away too much about The Mule - and neither does Asimov in fact - but I do believe that without the Mule, a man with strange capabilities, the Foundation series would be just another SF story.  Being an individual, already impossible to catch by Psychohistory, he also has the powers to turn him into a real threat for the Foundation.  The whole basis for the series is overthrown by this man, and it gives the second half of this book a tension the first half lacked.  And that tension is kept until the very last page.

If you liked The Foundation, you'll love this one.  And if you haven't read it yet, you'll notice that after reading this Foundation and Empire you'll want to find out all about the origins of the Foundation..