It's light-heartedly appalling. Then it's even worse. Then, ultimately, it's just funny.
Our hero, Hassaku, is having internet sex with his sister, Mao, who's studying abroad. He masturbates to movies she sends of herself saying how much she wants his semen.
Next thing we know, though, he's being bound and molested by topless girls! Hassaku's father founded the Amami Holy Mother's Academy (...so they're Catholics?), but the leadership has had policy differences. The school went co-ed last year, but Arisa (daughter of the current headmistress) and her friends have been assaulting and graffiti-ing boys to make them drop out. Hassaku's the last boy on campus.
I'm not convinced that having topless girls draping themselves on you would repel all teenage boys, but hey.
Anyway, Mao rescues Hassaku, because she came back a week early. She couldn't resist the call of her loins. "It's been so long since I smelled your semen!" She ties up Arisa, licks up semen, has a urination flood, makes Hassaku perform various acts on Arisa (sometimes lending a hand herself), etc. After Mao's 'training'... "Arisa-chan, you're now Hassaku's slave. From now on, you'll address him as 'master'."
By that point, Arisa's quite enthusiastic about that. She'd always secretly liked Hassaku. Touchingly, she's shocked to learn about Mao and Hassaku's incestuous relationship.
What makes this funny is the absurd levels to which it's taking the blood relationships. There are three Amami siblings: Mao, Hassaku and a younger sister (Airi) who'll presumably appear in ep.2. Arisa is their first cousin, since their mothers were sisters. The Amamis are orphans, but Arisa's mother is still alive and working as the school's headmistress... which apparently involves masturbating to the security camera footage of her daughter being tied up and used for incestuous sex. (Did she lift a finger to intervene? Call for help? Anything at all? Nope.) "Amami siblings, don't think this is over yet!"
It's less outrageous, so it's also less entertaining. It's a rerun of ep.1 with different characters, basically.
Hassaku's little sister, Airi, does indeed show up. She has a pain in her chest. A school teacher (in a nun's outfit, no less) diagnoses from this that Airi's jealous of her siblings and recommends the unusual medicinal treatment of sucking Hassaku's penis. I'm pretty sure teachers shouldn't be telling their pupils to do that. They definitely shouldn't be watching lasciviously, then joining in.
The headmistress calls Hassaku to her office. Mao shows up to save him, explaining that she "worked her way through the board" to make the school co-ed. I can guess what that means. She also compares Hassaku's penis with their father's, but fortunately with a question intonation. (The headmistress is the one who can judge there.)
The headmistress is an idiot, incidentally. She didn't turn off her own office's security cameras when performing sex acts in full view on her desk. Maybe she'd been masturbating to the sight of herself masturbating?