Yūko Mano

A.D. 2015: A virus has been spreading in many cities worldwide. It is a suicidal disease and the virus is infected by pictures. People, once infected, come down with the disease, which leads to death. They have no way of fighting against this infection filled with fear and despair. The media calls the disease the "Lemming Syndrome".

湖畔の別荘に集まった3組の家族と塾の講師は、中学受験を控えた子供のために勉強合宿を行っていた。参加者の一組の並木俊介と美菜子は別居中だったが、受験のために、仲の良い夫婦を演じていた。そこへ俊介の愛人が現れ殺害される。