Is it a western? Yes. Is it sci-fi? Yes. 500 years in the future, society is a mixture of "core" planets with all the luxuries and those on "the rim" where life is more like living in a old time Western. The Alliance, the totalitarian government, controls everything in the core and would like to exert the same control over all the planets.
Simon and River Tam are on the run from The Alliance. They take refuge on a spaceship whose crew will do anything, legal or illegal, to keep fed and in the air. As a survivor from the losing side in the recent civil war, the ship's captain, Mal Reynolds, doesn't mind going against the authorities but has to rethink his decision when the Alliance sends an assassin to track River down. What follows is a fantastic, fast moving adventure crackling with wit. It is also one that examines what different men choose to put their faith in, how strong that belief is, and what it means to live an unexamined life. (My complete review is here.)