Thanks to:

Circle: Ameshoo http://www.dlsite.com/maniax/circle/profile/=/maker_id/RG17424.html

Enjoy!

This is one of few hentai you need to know japanese to know what the hell is going on.

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Jesus this is some deviantart level artistic style

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Okay...if I had to guess (And I do, speaking no japanese), what's happening is that the futa stuck in the tub can only leave by getting someone else into the tub, and switching bodies with them (and drowning them maybe?). I won't guess if the sex is required, or the former futa was just an exhibitionist...

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Lemme translate then, I guess... whenever someone gets dunked in the tub, they swap bodies with the futa, so the futa was formerly inhabited by a male college student, who has now taken over the body of the tan girl. The tan girl gets stuck in the body of the futa, and once she cums she passes out and the guy who took her body gets away somehow. She starts to get really hungry for someone new to fuck, and in the last panel we see a guy has noticed her

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chenchen: Actually, the girl wasn't a futa to begin with. After the guy switched with her and explained the swap, she suddenly grew it. (Even he makes a surprised comment about it) You're correct on the rest, that the girl cannot leave the bathtub, and only a rare few can actually see them inside the tub. So she can't get her original body back, but she can swap with someone else.

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