One day, humanity was invaded by another world. Having experienced that battle, known as Other World War I, humanity moved to marine floats, built for emergency evacuation purposes, as soon as Other World War II began. It's around that time that Kizuna Hida is summoned by his elder sister and visits the Strategic Defense Academy Ataraxia where he meets Aine Chidorigafuchi, a girl who uses Heart Hybrid Gear Zeros to fight the otherworldly enemies. He has a tough time dealing with her, but when her Gear runs out of energy, the only way to get it back in action is to do something lewd to her!

[Source: Crunchyroll]

Masou Gakuen HxH specials included with the volumes of the DVD/BD.