The humans who managed this world managed it badly, and the accumulated pollution they produced has reached the point where the sun itself is considering leaving. An old legend describes a hero who once gathered sun rings from locations across the world and used them to call the sun back. No one has done this recently because it involves an amount of personal danger that discourages volunteers. Perhaps you are the exception — someone who loves the world enough, or is unhinged enough, to take the risk on its behalf.