In Japan, there exists a government agency known as the Supernatural Disaster Countermeasures Division (SDCD), whose duty is to protect the citizens from creatures unseen. They are able to dispatch these monsters swiftly and without alerting the general public. But currently, they face a different challenge: the betrayal of one of their own.

Shuntarou's younger sisters - Ayaka, Chiaki and Fuyuno - are very mean to him, so he wishes he had a gentle older sister. Then some mysterious game seems to make him younger.