On a rainy day nearly a month ago, Kumo Shirasagi went missing without a trace. Determined to locate her, a group of six classmates gathers to explore the Bird Forest, an abandoned theme park atop a secluded hill. Among them is Gin Kurobe, a mysterious boy who mumbles to himself with enigmatic motives. Their quest leads them to the looming Birdcage Castle, where they are suddenly rendered unconscious and awaken to find themselves bound by metal collars, forcing them to navigate in pairs without retreat. Breaking the rules spells fatal consequences. As they navigate the perilous castle, Gin and his companions face deadly challenges and psychological warfare in their desperate search for Kumo.