When Nube and his classmates arrive on a small island for a beach outing, they hear a frightening tale from an elderly resident. The legend speaks of a creature called the sea spider that inhabits the waters around the island. When the creature reaches 200 years of age, it transforms into a monster that devours humans. Curiosity kills the cat, so Yukime decides to investigate the truth of the legend. Suddenly, a girl appears on the beach. Possessed of an irresistible charm, she immediately captivates Hiroshi and overwhelms him. Soon after, strange events begin to unfold, and Kyoko and Ritsuko‑sensei are attacked under mysterious circumstances.