Riku was taking photos at the street corner when he met the self-proclaimed minor goddess Umi. He didn’t know if it was a fated meeting or simply planned, but she asked him to go to the past and change one particular event that happened back then. His assistance was needed since he contained the soul of one of the people involved in that event.
If he didn’t help her, what would happen? His surroundings and everything he took for granted will surely change, and misfortune will befall his childhood friend Yuika. This is because in Yuika’s past life as Chiyo, she killed her dear friend and her lamentation had persisted over the next 430 years, gradually eating away at her soul. The only way to save her was to avoid the incident that caused her regret, but this was the last chance to do so.