No matter what turmoils Nukumizu Kazuhiko goes through, he always knows his clingy little sister Kaju has his back. But one Valentine's Day, the unthinkable happens - she makes chocolate for everyone except her beloved oniisama! Something must be wrong, and Nukumizu's got to get the bottom of it. Maybe he could sneak into her school and pretend to be a junior high student? Or maybe he's just got to accept that he perhaps isn't the shining jewel of his sister's heart any longer...