Foundation and Empire

By now the Foundation has weathered the kingdoms at the galaxy's edge and grown into a power in its own right, still shielded by Hari Seldon's Plan. The first half of this second volume pits it against the dying Empire itself, when an ambitious general marches on Terminus in what looks like the gravest Seldon Crisis yet.

Then Asimov breaks his own machine. The Plan assumes history is shaped by mass forces, not individuals, so it has no answer for a single unpredictable person of enormous power who begins toppling worlds and bending minds to his will. As this new conqueror sweeps across the galaxy, the Foundation's faith in Seldon's mathematics is shaken to its core, and its survivors are left chasing rumors of a hidden Second Foundation.

Foundation and Empire is the pivot of the trilogy, the point where Asimov's grand deterministic scheme meets the wild card it never accounted for.