Years ago, a rare sea ore was discovered beneath an island. Named for its vibrant hue, "sea sapphire" showed potential as a powerful green energy source. To mine the underwater deposits, a prison was constructed on the island to utilize prisoners for the work. However, it became clear over time that prolonged exposure to the ore caused a profound transformation. The prison was subsequently replaced by a research facility dedicated to studying "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 crashes 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.