Beneath an island lies the valuable 'sea sapphire,' a remarkable green energy source. Initially mined by prisoners in a sea ore craze, the ore's dark secret emerges - it induces a strange affliction known as 'indigopathy.' The prison shifts its focus to research on this mysterious disease, abandoning mining. Awakening disoriented in a research facility, a man encounters a blue girl, a fellow patient, bursting into his life. Struck by her azure features, he is intrigued by her familiarity. Known as 'Kai,' she rallies him to flee the island to honor a forgotten pledge.