I think that the story is interesting not only because of the premise behind it (going back in time to change the things you regret) but because of what it could eventually lead to.
We start off on graduation day where our main character, after looking back on his high school years, despairs that he didn't accomplish anything and regrets not living his life to the fullest (joining clubs, making friends, etc). After graduation, he enters and old building and runs into a classmate (who I'll refer to as A-ko) and, somehow, the two end up travelling back in time to the start of their high school careers.
Comparing the 10th grade main character to the graduate we originally met, we can see very distinct differences in their situations, etc. This, for me, is where the potential for this story lies. Although the main character begins merely by joining a club he regretted not joining, who's to say he can't use his knowledge of the future to create an entirely different life? We can see a bit of this with A-ko as she attempts to rebuild the life she had upon graduating 3 years too early.
For me, this manga raises an interesting moral question: if given the chance to go back in time and relive a part of your life, what would you change? Would you use your knowledge of the future to make yourself a billionaire, or prevent a loved one from dying, or perhaps try to recapture your original life like A-ko? Would you do it even if it meant creating an entirely different future that could affect everyone around you? I'm really interested to see what the main character chooses to do in his redo and how that will affect the natural flow of time.
