Kako no Anata, Mirai no Kimi

Sayako has been in a relationship with Naohiko for 10 years, but the passion of love faded since long and their couple starts to slowly crumble. But one day, Sayako finds in the street a stair that she never saw before. Taking it she comes back 10 years in the past, when Naohiko was still a teenager, in a world where her teen self doesn't exist.

This is the start of a strange love-triangle between Sayako, Naohiko as an adult and Naohiko as a teenager.