Thunder 3 Volume 1 (manga)

Pyontaro is a middle schooler whose little sister, Futaba, simply adores him. One day, he and his buddies—the so-called “Small Three”—decide to watch a weird disc their teacher lent them. Little do the boys realize this disc is linked to another universe...and while they aren’t looking, Futaba gets sucked in! Can Pyontaro and his friends find Futaba? And just what is this other world they've discovered?!