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HoloEN in a private onsen! Everything is fine, and the girls are having fun soaking in the springs. After some time, their minds begin whiting out, surely because the water is so relaxing. Next thing they know, it devolves into an orgy!
Hentai logic: everything is the fault of one janitor who added some floral concentrate into the water, not knowing that with Fauna's sweat and mucus it would form a powerful aphrodisiac. Fauna would be affected by it, become wet and in turn form even more of the aphrodisiac, eventually affecting everyone else in the bath!
Hentai coincidence 2: janitor comes back to see the exhausted girls, reasonably assumes heatstroke and tries to call for help, but they swarm him, and get him drunk on aphrodisiac as well, with predictable results.