Seventeen-year-old Aoi Ryugoku has always felt out of place, her sapphire-blue eye and mysterious blue crystal necklace marking her as different. When the long-hidden Guardian Spirits of the world converge for the first time in 500 years, Aoi must navigate a high school filled with peers who unknowingly carry ancient powers. Alongside her soul sister, Mao, and guided by a few who know the truth, she begins to uncover which classmates are vessels for these powerful Spirits. Friendships will be tested, secrets revealed, and the balance between human and Spirit will shift in ways no one could anticipate.