From first chapter:

The main character is a student attending his high-school graduation. Due to his appearance and personality, he didn't make a single friend during his three years of high school. As he remembers his days in the school, he wonders what his life would have been like if he had made different choices. In particular he wonders about his life had he answered differently when girl from the cheering club asked him to join. Since the cheering club was disbanded, he feels he should take one last look at their club room before leaving the school forever. However, he is misunderstood by another girl that was waiting for her boyfriend and as he chases her (to clear up the misunderstanding), she falls down the stairs (possibly dying). He goes to check on her but ends up falling too. He wakes up to find he is back to his first day in school (three years ago). Thinking it's obviously a dream, he just tries to correct the mistakes he made during that day...until the girl shows up blaming him for pushing her down the stairs and what's worse bringing her three years into the past!