On a hot summer day, a young boy named Yuta visits the dam where his father died one year ago. At the site he meets a mysterious old man. He gives the old man something to drink and later that day the old man saves him after slipping off the dam. He wakes up to discover he is 30 years in the past. There he meets a girl named Saeko, who loves fireflies, and also befriends a boy his age named Kenjo.