The actual story isn’t really for vanilla lovers, since we see how mind breaking works on Boa Hancock. However, if you love a happy end then you won’t be disappointed by this hentai manga (neither story/nor comedy wise).
Now back to the plot. The snake empresses Hancock is bound in chains and serves as a sex slave for the Tenryuubito (world noble in One Piece universe). At first she is somewhat resisting, but after some time she opens up starts enjoying sex and thus she becomes a filthy whore who orgasms by the mere look of an erect dick.
Like I said, read the last page and you will piss your pants from laughing 😉