I’m not sure what to say about this book that I didn’t say about the last book. This one’s like that one, only more so.
It’s divided into two parts. In the first, what’s left of the Empire is at war with the Foundation. (You’ll either like this, as it shows the Foundation a capable successor, or you won’t like it. Is it just to wage war now even though people will die, if there’s a 94.3% chance that doing so will radically reduce the length of the dark ages?). The Foundation eventually prevails thanks to Hari Seldon’s recorded (centuries old) instructions on what to do in this precise circumstance at this precise time.
In the second, we meet the Mule, who apparently possesses some sort of genetic mutation that allows him to manipulate the thoughts and emotions of others. He is using this power to wage war against the Foundation. Unfortunately, Seldon doesn’t seem to have predicted this exact situation and so, the Foundation is in peril. Perhaps Seldon’s calculations didn’t account for genetic variation. The only hope is the Second Foundation.