Puberty syndrome – Abnormal experiences rumored on the internet to be caused by sensitivity and instability during adolescence. This year, Sakuta Azusagawa, a second-year student at a high school near Enoshima, meets several girls that are experiencing this “puberty syndrome.” For instance, he meets a wild bunny girl in the library. She turns out to be an actress on hiatus, Mai Sakurajima, who is also his senior at the school. For some reason, no one else can see this enchanting girl. How did she become invisible…? As Sakuta searches for answers to help Mai, they spend more time together, and he eventually learns of her hidden feelings… This is an unusual story that takes place in a town of bright skies and glistening seas, where Sakuta encounters various intriguing girls.

While looking for the cause of Mai's Adolescence Syndrome, Sakuta ends up scheduling a date with Mai. On the day they promised, Sakuta tries to help out a lost little girl on his way towards Fujisawa Station. But, he is misunderstood to be a perverted maniac by another girl, and because of that, he ends up arriving late to his appointed time to meet Mai. He somehow gains Mai's forgiveness and they proceed with their date, but Mai changes their destination, and takes Sakuta to Shichirigahama Beach...

Even after leaving Fujisawa for Ogaki, not one person remembered anything about Mai Sakurajima. Sakuta and Mai return to school together, but even there they hear from Sakuta's friend Rio Futaba that nobody at school outside of her has any recollection about Mai. At the same time, Sakuta hears of a theory that losing memories concerning Mai is based on a certain action taken, and struggles so that he doesn't lose his memories of her. And seeing Sakuta act as such, Mai makes a certain decision of her own as well...

Soon after Mai's "puberty syndrome" is resolved, Sakuta encounters another strange phenomenon without respite. Day after day, it is the same day for Sakuta and there is no tomorrow. When he consults Rio, she raises the possibility of "Laplace’s demon" -- that someone may be repeating and redoing a day.

Because of the misunderstanding that she was dating Sakuta, Tomoe is able to resolve the agonizing situation of being asked out by the guy her friend is in love with. She pleads with Sakuta to continue acting like they are dating. Despite feeling indebted to Mai, Sakuta could not leave Tomoe alone as she was fearing that she would lose her presence within her class. So, he reluctantly agrees to play the role of Tomoe's fake boyfriend for a while.

It is now the last day of the first term, the promised day when pretending to be dating Tomoe ends. It seems as though everything has finally been resolved, but…