Years ago, a rare sea ore was discovered beneath an island. Named after its vibrant color, "sea sapphire" showed potential as a powerful green energy source. To mine the underwater deposits, a prison was built on the island for the purpose of using prisoners to do so. However, it was revealed over time that prolonged exposure to the ore caused a profound transformation. The prison was subsequently replaced by a research facility devoted to the study of "indigopathy," the disease that slowly stains the human body.

One day, a patient wakes up strapped to a table with no memory of who or where he is. Just as he starts to panic, a girl comes crashing through the wall to his rescue. He is transfixed by the sight of his savior, whose hair, arms, and legs are all completely blue. But even more important than her appearance is the fact that she seems to know him. Calling him "Kai," she urges him to hurry and follow her so he can fulfill his promise: to escape the island together.