During a school trip to a remote island for some beach fun, Nube and his classmates stumble upon a chilling local legend about a sea spider lurking in the waters. Despite the ominous warnings, Yukime becomes intrigued by the tale and delves deeper. Their peaceful outing takes a sinister turn when a captivating yet mysterious girl appears, captivating Hiroshi and raising suspicions among the group. As inexplicable events unfold, both Kyoko and their teacher, Ritsuko, fall victim to bizarre attacks. The enigmatic girl's true nature is called into question, casting a shadow of doubt over the ancient legend of the sea monster.