Aqua, the Goddess of Water and Anal Sex. Convinced that anal pleasure is the ultimate form of divine ecstasy, Aqua has redesigned the female body in her image: all women who worship her (or simply travel in her party) are blessed with a permanent transformation. Their vaginas vanish entirely, replaced by fully functional penises and testicles, leaving the anus as their sole, hyper-sensitive sexual orifice - engineered to deliver pleasure far beyond anything conventional anatomy could offer.
Kazuma, hoping for a useful goddess this time, ends up saddled with the most shameless one yet. Aqua's "blessing" instantly affects Darkness, who becomes euphoric at the prospect of new depths of masochistic humiliation; Megumin, who rages at first but soon discovers explosive new sensations; and eventually every female companion they meet.
Now leading a party of futanari bound by their goddess's creed, Kazuma must battle the Demon King while navigating endless lewd misunderstandings, involuntary erections during combat, and Aqua's incessant preaching about the superiority of anal divinity.