Urashima, an island far from the mainland. The people who live there lead carefree lives. But five years ago, the island's three great families suffered a series of misfortunes, and succumbed to suspicion. The people of the island cut off all contact with the mainland, and began a slow decline. The key to saving the island lies in three girls who belong to the three families. But they are bound by old traditions, and are conflicted. On that island, a lone man washes ashore. The man claims to be from the future, and he begins a solitary struggle to change the island's fate.
(Source: Anime News Network)
— Description from AniList community (anilist.co).
