It’s an all POV video, where the protagonist is engaged with the girlfriend (now fiancee) Kamihata Ichika, and they promised each other to no have pre-marital sexual relations.
So the dude, still a virgin, gets to meet with Ichika’s family. Much to his surprise, all girls. And not only that, all of them are substantially pretty, even the Mom, Ayumi Shinoda.
Then he stays for dinner and he gets invited to stay overnight. When all the girls eat him alive in a free-for-all fashion, basically.