Very satisfying old Asimov, with a solid plot and good action that moves along. My only complaint is the rather drawn-out ending that just seemed to take too long to come to the point, along with Asimov’s trademark idiotic character logic.
I won’t give away anything, but the climax is absurd–with even the threat of such a calamity, the imperial navy would have simply quarantined Earth, or bombarded it. An empire of 200 million planets certainly could afford a few HUNDRED warships to that effect.
Instead, Asimov goes through an absurd amount of fencing between characters that dragged down a great story and made it tedious. Also, I read this one by mistake first; will correct the mistake by picking up the other two asap.