Soap opera style arranged marriage netorare, with blackmail sprinkled in.
------------
Taku and Rio live their school life as close sweethearts, eventually becoming lovers, Rio and Taku give their first time to each other, pure couple as it gets. Then Rio spends several days absent from school, Taku passes by Rio talking to a man he has never seen before, he is dropping Rio off after giving her a ride.
Taku and Rio talk, he is very uneasy now asking what Rio has been up to after missing school for that long, Rio says she did message him she would be missing school but Taku wants to know why, and now that he saw Rio with a stranger he wants to know who he is; Rio is looking absolutely sad through all of this, she just cuts to the chase, telling Taku how happy she was that she could at least give her first time to the person she loved, Taku doesn't understand it but Rio just says this is goodbye, that man is her fiance Hiroto, they were promised to marry each other after she graduates, sorry but this has been settled forever ago.
Taku can't process this insanity, pleading Rio to please think about this, there's no way this should happen, but she insists he let it go and forgets all about everything.
A short while later Taku is at school, looking down on the slumps, his buddy remarks it sure has been a while since Rio came to school and Taku cuts him off, telling not to say that name but his buddy insists saying Rio always acted like a bundle of joy, there's no way whatever she is going through right now is right.
Hiroto and Rio are together, he doesn't like how Rio is acting around him, so cold and distant, Rio says this is fine, they are to marry but why would she pretend to love him, Rio knows Hiroto is a playboy who screws around with women, she doesn't like unfaithful men, but Hiroto seems to be a believer men like him are what's normal, if other guys were in his position (rich and handsome) they would just be taking women left and right, Rio rebukes him, saying that's not true, dedicated faithful men exist.
Whatever, Hiroto entertains that yeah maybe men like that exist but Rio only has him, and even though he is a playboy he has feelings, he doesn't want his future wife to treat him like that so he wants her now, Rio forces him away which makes Hiroto immediately say that's not cool, Rio should know who would get in trouble if their marriage gets called off... this automatically make Rio stiff, completely opening herself to Hiroto.
Indeed, Taku's buddy actually looked around and heard from his family that all of this was a cruel fate decided upon Rio, her father's company went through a disastrous event leading them to bankruptcy but a much larger company went to the rescue, saving the company and promising to leave the company somewhat an independent subsidiary but under one condition: the scion of the larger company gets Rio's hand in marriage.
Taku is briefed on this by phone, which reinvigorated his desire to be with Rio, apologizing to her in his mind for being such an idiot, he should have stuck by her side no matter what, even if he didn't know all details there's no way Rio was happy with breaking things off like that, thus Taku runs to meet Rio and do whatever he can by her side.
There Rio is enduring Hiroto the only way she can, by holding onto the belief sex only feels good with the person they love, surely this thing with Hiroto feels disgusting, but he wants to taint Rio and prove her wrong, step by step Hiroto uses all his skills acquired with much experience to make her feel pure bliss.
Taku reaches the place, he tries to contact Rio, to no avail, but he manages to get inside, and the horrors of the moans echoing reaches his ears, Taku doesn't want to believe it but that's is definitely Rio and likely Hiroto too, Rio held off as much as she could but Hiroto knows his way around a woman, she is feeling good and Hiroto demands she says it out loud.
Unknown to her Taku hears it, and that completely destroyed his renewed wish to be by her side, if it appears she is loving her actual promised man then he is just an outsider.
Hiroto praises Rio for being honest, saying he will take care of her from now on, Rio is ashamed of herself for giving in, tightly holding the little mascot Taku gave to her as a lasting memory of the love she had to leave behind, THE END.