While on her high school senior class trip to the famous Madooka Caves, 17-year-old 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 Miss Dimeeners among others and comes into contact with 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 who the Little Citizens call Iyamsar, and he hatches an evil plan of obliteration. She must betray and endanger the Little Citizens or keep silent and risk losing the opportunity to escape and possibly endanger her very life. Will the wicked ruler's only son come to her aid or will he continue to serve his father?