The protagonist, Keisuke Shinozaki, returns alone to his grandparents’ house to stay for summer break.
There, he is greeted by his mother’s (much) younger sister, Yuuka.
Ever since he was little, the protagonist had always adored his kind, smart, and beautiful aunt.
However, it was quite the shock to learn that she was now engaged…

Both “Longing” and “Regret” lead down two different paths.