One of those romanticized affair stories, it can be very cute depending on how reasonable you think these settings are, or you might think they should have broken up first before the new romance can take place; it's a matter of opinion and morals.
----------
A gyaru called Miku is having sex with her boyfriend Kazu in a love hotel, they finish and Miku asks Kazu if he has any grand plan for the weekend, she is fishing for a specific answer but Kazu doesn't seem to get it, he just says he doesn't have anything planned.
On their out Miku and Kazu are accosted by a stranger who gets all worked up upon seeing Miku, he oddly starts calling her Misaki, desperately asking how can she be here in front of him, she is supposed to be dead! Naturally Miku is pretty creeped out by this stranger just grabbing and rambling about whoever this Misaki is, Kazu rushes to protect his girlfriend, pushing this crazy guy away, Kazu notices they are making a scene with onlookers taking a glance at them so Kazu takes Miku and leaves.
Afterwards, Miku is calling Kazu to once again ask what plans he has for the weekend, he still says there's none, that's where Miku gets direct with him, expressing her disappointment over Kazu not caring about their one-month anniversary since they started dating, she really was expecting they would have a special celebration but Kazu doesn't seem to care, thus alone and vulnerable Miku is approached by two playboys who saw a target for their trying their shot, they aggressively ask Miku to go out with them, actually starting to molest her; however, the strange guy from before of all times passes by and noticed Miku was in danger, he rushes to help her, telling the sleazy dudes to back off, can't they see she doesn't want them? He gets beat up but once Miku sees an opening she kicks one of them on the nuts, she grabs her surprise knight and runs.
They go to his home, Miku feels indebted to the stranger, no longer being weary of him, he introduces himself as Toru; Miku feels indebted and wants to thank him for saving her, Miku takes a glance at a picture frame with Toru and a girl that looks very similar to herself, Miku wonders if that's why Toru called her Misaki before? At any case Miku starts blowing a reluctant Toru but she gets a sudden call, it's Kazu who just casually cancelled their date... a vulnerable Miku just unloaded her frustrations with Kazu on Toru, she is seriously sad over her boyfriend being so uninterested in celebrating their one-month anniversary, Miku worked harder on her job to earn enough to really spend in this special occasion by reserving premium tickeds at the carnival but Kazu doesn't give a damn, it's like only Miku is treating their relationship as something that she expects to be truly solid and lasting, this conversation takes most of Miku's free time so she leaves Toru for the day, but she wants to meet him again.
Miku ends up taking Toru to the planned date she would have had with Kazu, and to Toru's surprise Miku styles herself exactly as his deceased girlfriend Misaki, Miku thought he would like it, anyway, they have their date exactly as Miku planned, for the final part Miku takes Toru on the ferros wheel, saying Toru will win a prize if he hits the right marks, Miku initiates a heart to heart with Toru, saying how things are turning the way she expected with Kazu so maybe if went out on a date with Toru, becoming the girl he once loved maybe the attention Toru gave her would take Miku's mind off things... Toru just apologizes to Miku for seeing Misaki on her, Miku is her own person, that isn't right, he is having fun with Miku right now for who she is!
That moved Miku's heart, making her want to fully give herself up to Toru, they go her home this time, Miku still wants to entertain the Misaki fantasy but when having truly passionate sex Toru called Miku by her own name, this struck a chord in Miku's heart. When they seemed to have finished for the day, Toru thinks the night is over but Miku takes her wig off and opens herself up fully to Toru, asking him to please love her as Miku, but her phone rings, Toru thinks it might be her boyfriend Kazu but Miku doesn't care she wants Toru now and so they have a much more passionate sex with Miku's thoughts being all about how Toru might just be the loving person she wanted all along, the one she will truly devote herself to.
Miku and Toru spent the whole night together in pure loving bliss, the phone isn't ringing anymore, Miku wakes up and Toru happily says good morning to Miku, emphasis on her calling her name which pleases Miku a lot, THE END.
----------
I assume Miku is just over Kazu at this point and has implicitly broken up with him by just ignoring the last call and having her last thoughts be about how Toru is the man she dreamed of, I think it should have been shown more explicitly though, a very cute relationship started with Toru and Miku but the air of having an affair was still there.