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.
When he wakes up, Setsuna has no memories. He is on the island left behind by time, Urashima. Everyone he meets is mysterious and puzzling. At night he is surprised by a girl singing on the beach, and somehow says her name. “Rinne…”
Setsuna begins to research soot blight syndrome to understand the shocking statement Rinne makes. The endemic disease keeps the island isolated and toys with Rinne, Karen and Sara’s fates. But Rinne’s mother Kuon knows something unexpected.
Setsuna, Sara and Karen try various ways to get Rinne to come out during the day. Their fun time is interrupted the moment when Rinne sees a hidden shack. After that, Rinne tries to stop thinking about the past, but Karen makes a decision to face it down.
Although it was by Karen’s request, Setsuna does something unthinkable. They go to the mainland so that Karen can escape and go to see her mother. Why did she cry while Setsuna watched over her? Meanwhile, Sara is getting close to something.
“I don’t have time to spare!” Sara yells hurriedly. She has a hypothesis as to why Setsuna came back in time and that he and she are destined to be together, but Rinne confronts him with a dark truth. Crimson flames lick the heavens, bringing an end to a bloody history.
The darkness has lifted for Karen and Sara. In bed, Rinne murmurs, "Next time, will you be my Setsuna?" She and Setsuna enjoy a day of happiness together. Rinne has prepared herself bid farewell to the past and set off toward tomorrow. But Kuon seems to have misgivings.
Rinne and Setsuna thought they had left their pasts behind. Then, Rinne is a horrible memory she cannot ignore. Meanwhile, Setsuna is faced with a new revelation. The situation changes suddenly as Rinne, believing new information she just learned sets out into the stormy night and raging sea, and disappears.
When they awoke, they were looking up at the blue sky from five years ago. It was the mysterious island where Rinne and the real Setsuna had been before parting ways forever. Rinne feels excited to see the real Setsuna again and finally settle things, but what they find are more riddles, tears, and darkness.
In an underground shelter-city encased in ice, Setsuna meets a young girl named Rinne and begins helping her with a mysterious invention. It can apparently help save the world, but its true nature is unclear. Meanwhile, the city itself will very soon perish.
Setsuna rises up to create a future for a city without hope, but tragedy strikes time and again. Meanwhile, Rinne finishes her invention and Setsuna sets out for the future to save mankind’s existence. Just before he goes, Setsuna makes a promise to Rinne.
When Setsuna opens his eyes, he’s back on the same beach in the summer. Everything happens just as it did before. Setsuna rushes to reach the one moment he wants to change. But he learns learns a shocking truth.
Rinne and Rinne... A truth about these two girls is brought to light by a photograph, and Setsuna makes a decision. Will it make everyone happy? A new future awaits once this great mystery is solved. No more hesitating. It’s the eternal finale.