Almost a month ago, on a rainy day, Kumo Shirasagi vanished without a trace. Hoping to find her, six classmates gather to search the Bird Forest, an abandoned amusement park situated on a remote hill in the countryside. Among them is Gin Kurobe, a boy whose reasons for joining are as mysterious as his habit of muttering under his breath. The group's search leads them to the Birdcage Castle, a foreboding structure looming over the park. In an instant, they are knocked unconscious—and upon waking, they discover metal collars around their necks, each linked to another person. The game they have unwittingly entered is simple: they cannot retreat from any chosen path, and they must always move in pairs. Breaking either rule results in certain death. With their lives at stake, Gin and his classmates must ascend the bloody castle, enduring the psychological traps set against them while searching for Kumo.