Lucy is a Sleepwalker, a rare person with the ability to travel within the dream world and collect magical items to sell to the nobility.
When a darkness consumes the dream world, it corrupts everything it touches - turning the dream-creatures into nightmarish monsters and tainting Lucy's enchanted wares. To make matters worse, the prince of the real world, Joon, vanishes overnight. In his absence, the darkness spills out of the dream world into the waking world, threatening to suffocate Lucy's people under the weight of nightmares and paranoia.
When Lucy discovers that a creature stalking her is really a cursed Prince Joon, she realizes the darkness is specifically targeting the prince and his country. Banding together, Lucy and Joon fight demon dogs, grinning ghosts, and the darkness itself in their quest to restore the light.