Dude stumbles upon a cursed jar that dooms him for eternity unless he impregnates some bitches. And it just so happens, he has three step-sisters willing to help.
This dude Yoshiyuki loves to work out. He also loves sex, cause he's not dead. Now that he's a teacher at an all-girls school, he gets to combine his two favorite things. He decides to give his struggling students some private tuition, AKA, his dick - cause that certainly won't be hard to grasp.