Long ago, demons preyed on humans, feeding their souls to their ruler Gwi-Ma. Eventually, three women became demon hunters and sealed the demons away with a magical barrier called the Honmoon. Their legacy continued across the generations, with each new trio using their singing voices to maintain the Honmoon, working to eventually strengthen it into the Golden Honmoon – a final seal that would banish demons from the human world permanently.

In the present, the K-pop girl group Huntr/x – comprised of Rumi, Mira, and Zoey – is the latest demon-hunting trio under the guidance of Celine, a previous hunter who raised Rumi. After completing their world tour, Huntr/x release and prepare for the first live performance of a new single, "Golden", until Rumi begins losing her voice due to her secret demon heritage, which only Celine knows about. As a child, Rumi was told by Celine that turning the Honmoon gold would erase her demon marks.