After working through the night to finish work, Yoshino and Yuu stays behind together. Yuu saves Yoshino from falling over the trash can, and starts to tease him by tickling him. Hatori walks in on the scene and misunderstands. Later, Yoshino arrives at Hatori's house and attempts to apologize, but gets mad at Hatori when Hatori says he does not want Yoshino to have physical contact with Yuu. They make up temporarily in the morning after Yoshino sleeps over, but gets into a dispute again when Yoshino reveals that he is going to a hot spring with Yuu. On the night of the hot spring trip, Yoshino is surprised to find out that it is his own birthday on that day and realizes why Hatori is upset about his going away. Briefly after his realization, he receives an unexpected love confession from Yuu, but is interrupted by a phone call from Hatori. After hearing a "Happy birthday" from Hatori through the phone, Yoshino suddenly decides to terminate the vacation and go back to Hatori's house, where Hatori gave himself to Yoshino as a birthday present.
Sekaiichi Hatsukoi follows three couples that are interconnected within the manga industry, with each being subject to the budding of first love.
