Mysterious blue fireflies have slowly made their way through a crack in the protective dome over the city of Refugia. Although they appear harmless, a single touch can transform a human into a disfigured, mindless monster. Despite providing their citizens with comfortable lives and the latest technology, Refugia's controlling government conceals this danger from the public.

Four young girls—Sena, Dabi, Yojae, and Joan—join a unit entrusted with testing a new security system. But when they come too close to the confidential secrets of Refugia and have an unfortunate encounter with the blue fireflies, they are magically teleported outside the dome. Only vaguely aware that this is their first breath of true freedom, the girls decide to investigate the world beyond.

Guided by a hidden clue, the girls search for the mystic Blue Firefly Island and the secret behind a magical necklace that comes into their possession. Armed with optimism, the girls embark on their first-ever adventure, where the outside offers revelations about Refugia and its corrupt rule—as well as dark truths that will forever change the world as they know it.