From Chibi Manga:

Let’s get into a time machine and return to that time.
Where it was always fun and you were always laughing, to that time...
"I only have three more months to live"
Yui who just transferred to a new school in the countryside becomes Ryo’s classmate. Yui who is suffering from an illness has a "wish" that she can’t tell anyone, but Ryo wants to grant her that wish but…
After death, reality hits Ryo and Yui.
This is the story of the eternal promise that was made by the two of them.