Kako no Anata, Mirai no Kimi

Sayako has been in a long-term relationship with Naohiko, but their love has grown cold, and their bond is beginning to break down. One day, she discovers a mysterious staircase in the street that leads her back in time ten years, to when Naohiko was still a teenager. In this new timeline, her younger self does not exist. As she navigates this altered past, a complicated situation arises involving her adult self, Naohiko as an adult, and Naohiko as a teenager, forming an unusual love triangle.