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.
Kissing isn't the only thing lovers do.
Besarse no es la única cosa que los novios hacen.
Besarse no es lo único que los novios hacen.
Beijar não é a única coisa
 que namorados fazem.