Kojima Arata is just a normal second year high school student. One day, while going to school, he comes across a girl from his past. Back when Arata was only a child she saved his life from a speeding truck. Now, his saviour stands once again in front of him. As he approaches the girl to thank her for saving him that day, something strange draws Arata's attention—her appearance is the same as back then, like she hasn't aged at all. However, as soon as Arata talks to her, the girl breaks into a run. Arata chases after her, but she manages to escape by entering a dead-end alley, leaving a confused Arata wondering what happened. She was just gone, vanished like she never went in there. After searching around for a while, Arata gives up and heads for school.

Two of Arata's best friends await him there—Shoji Hayato and Uehara Naomi. He has known them since the first year of high school. Later that day, another girl joins the group of three. Takahashi Erika, who just transferred to their class that day, quickly becomes friends with them. At first, she looks like a nice girl to Arata, but he soon learns that his first impressions were wrong, as Erika keeps teasing and playing tricks on him every time she gets the opportunity.

What will happen to Arata from now on? Will he be able to find that girl from his past and finally thank her for saving his life? Will he keep enjoying his everyday life with Naomi and Hayato? Or will this new girl, Erika, turn that normal high school life into something extraordinary?