Shun's younger brother Fuyuki and his girlfriend Mamiya-san kissed.  When Shun finds out, he asks his friends what he should do as Fuyuki's older brother.
Fuyuki ignores his brother and goes on a date with Mamiya.  The gang decides to follow the couple on their date and end up at a karaoke box.
You were acting so happy every day 
just because you'd kissed Mamiya-san.
Estabas tan feliz por haber besado a Mamiya-san.
Estabas tan feliz por haber besado a Mamiya-san.
Você estava tão feliz no outro 
dia só por tê-la beijado.