Prologue. Hourglass. Summer. Three young heroines walked in the yard. In today's sunny day, no trouble foreshadowed. During a walk together, Mao decided to swing on a swing. Ashley and Yoku sat down on the bench to talk. Suddenly, someone called out to Shiroin, and the girl immediately jumped off the swing into the sandbox, and... drowned in a sand funnel. The girls sitting on the bench ran immediately to the scene, becoming carefully studying it...