The galaxy is trapped in an endless cycle of extinction. Every 50,000 years an ancient machine race invades and with ruthless efficiency wipes out all advanced organic civilization. They leave behind only the scattered ruins of technology, destroying all evidence of their own existence.
Best game of all times. Even better than Half-Life (shame on me for saying this).
The story is extraordinary well written. Characters are acting even better than some real-life people.
And as a cherry on the cake - most of your actions actually affects the game world.