Time is something that flows in one direction. From the past to the present. But what if the direction of flow is different? Because the past can only exist if there is a future.
This concept starts to make sense to Natsuki, when she meets Tatsumi Shuuya. 15 years ago he saved her life. She was kidnapped by some people by a mistake, and just as they wanted to kill her, she got saved by him. In her recent life, only her panic attacks and the inability to stay in tiny rooms remind her of that incident. But now Tatsumi showed up again, not only telling her she will fall in love with him, but also not looking a single day older than at the time she met him for the first time.