Black Gate

Narifumi Hosokawa prepares for his high school's culture festival with all the Kadomori twins, Shizuku and Kasumi. While everyone is distracted, Shizuku decides to spread out a mysterious gate by making use of magic and travel to a different world. She has broken the oath of her family's heritage, regarding protect the Black Gate from those that seek the enormous power that lies within the world. The remaining defenders in the Black Gate, consisting of female students and a female teacher, must cross to the other side and return before any distruptions arrived at either dimension. However, Narifumi accompanies the gang and Shizuku delays for them with a few new allies.