Ex-soldier Sunho lives in the Under World, a land of perpetual darkness. Possessing just his name and sword, he comes across the score of a lifetime - a chest of coins for hunting down the girl who wields silver light.

Ren is a spirited acrobat travelling with her family. But everything changes when they are attacked by a demon. Desperate, Ren releases a blast of silver light and kills the monster - but cannot save her beloved uncle from grievous injury.

Determined to save him from succumbing to the poisoned wound, Ren sets off for the mountains, where the creature came from - where Ren herself fled from ten years ago. Her path collides with Sunho's, but he doesn't realize who she is. As the two grow closer, it becomes clear their pasts - and destinies - are more entwined than they could possibly have imagined . . .