Kon Chigawa is a lonely middle school student living on a small, isolated island she has never left. Shunned by her classmates and unable to relate to their talk of falling in love, she spends her time alone, often lingering near a seaside cave the locals call "the Hole." Folktales say a man-eating monster dwells inside it. One day, she encounters a city girl named Furuka Hitoto on a cliff. The two fall into the ocean, and Kon manages to rescue her. When they regain consciousness, they find themselves inexplicably standing in front of the monster's cave. Intrigued and bound by their chance meeting, they decide to enter together.

As Kon and Furuka explore the cavern and the island itself, their bond deepens. The presence of the monster—real or imagined—begins to trigger strange occurrences around them. In the second major arc, the pair spot a faint silhouette near the monster's lair and venture deeper inside to investigate. Their escape route is cut off, and they become separated, leading to escalating danger. The story weaves together the growing emotional connection between the two girls with the mystery of the island's supernatural legends, forming a fantasy coming‑of‑age tale centered on loneliness, first love, and the unknown.