The Currents of Space / Isaac Asimov,

Classic science fiction adventure in the Foundation universe by grandmaster Isaac Asimov. Only one man can warn the planet Florinia that it is doomed by an astronomical catastrophe -- but he has amnesia and is a slave.

High above planet Florinia, the Squires of Sark live in unimaginable wealth and comfort. Down in the eternal spring of the planet, however, the native Florinians labor ceaselessly to produce the precious kyrt that brings prosperity to their Sarkite masters.

Rebellion is unthinkable and impossible. Not only do the Florinians no longer have a concept of freedom, any disruption of the vital kyrt trade would cause other planets to rise in protest, resulting in a galactic war. So the Trantorian Empire, whose grand plan is to unite all humanity in peace, prosperity, and freedom, has allowed the oppression to continue.

Living among the workers of Florinia, Rik is a man without a memory or a past. He has been abducted and brainwashed. Barely able to speak or care for himself when he was found, Rik is widely regarded as a simpleton by the worker community where he lives. As his memories begin to return, however, Rik finds himself driven by a cryptic message he is determined to deliver: Everyone on Florinia is doomed the Currents of Space are bringing destruction. But if the planet is evacuated, the power of Sark will end-so there are those who would kill the messenger. The fate of the Galaxy hangs in the balance.