While on her high school senior class trip to the famous Madooka Caves, Serendipity Daniels becomes separated from her three best friends when after the four of them sneak away from their tour group, she alone sees a flash of the Lady in Blue and turns in the opposite direction to follow her. Sere is subsequently swallowed up in a labyrinth of caves and cave-dwellers unknown to the outside world where she encounters the paranormal and experiences enchantment, mystery, sorrow, friendship, and paralyzing fear. She makes the acquaintance of Muzzle Guzztree, Miss Drex, and others, and becomes uncomfortably aware of the Vent Whisperers. Her newfound friends aim to help her evade the clutches of the vile Enfatsoo and return Sere to her classmates, but they are too late. With the aid of his troglobites and an unwitting accomplice, the malevolent leader has learned of the intruder and hatches an evil plan of obliteration. Sere must now betray the Little Citizens or keep silent and endanger her own life while forfeiting any chance of escape. Will the wicked ruler’s only son step out of his comfort zone to come to her aid or will he continue to do his father’s bidding?