"The Case of Yoshiyuki Hatori" is the second OVA of the anime Sekai-Ichi Hatsukoi. It was released just over a week before the first episode of the second season.
Yoshiyuki Hatori and Chiaki Yoshino have been going out for a year now, and Hatori reflects over that. Yoshino gets a call from his mother who complains over how he never visits. As Hatori plans to go to his parents’ house, Yoshino decides to go along with him and visit his mother and sister. While there, Mrs. Yoshino makes a remark how Hatori and Chinatsu (Yoshino's sister) would make a nice couple, which makes Yoshino jealous. Hatori is touched by how much Yoshino cares for him.