On a rainy day almost a month ago, Kumo Shirasagi disappeared without a trace. In hopes of finding her, a group of six classmates convenes to search the Bird Forest, an abandoned amusement park on a remote hill in the countryside. Among these students is Gin Kurobe, a boy whose reasons for coming are as mysterious as his habit of muttering under his breath.

The group's search takes them to the foreboding structure looming over the park known as the Birdcage Castle. In an instant, they are knocked unconscious—and when they awaken, they find metal collars around their necks, each connected to another person.

The game they unwittingly entered is simple: they cannot turn back from any chosen path, and they must always move in pairs. The punishment for breaking either of these rules is certain death. With their lives on the line, Gin and his classmates must ascend through the bloody castle, battling the mind games thrown at them while searching for Kumo.