From here:

As a child, Morisaki Shinichiro made a promise to a girl when she was about to move away. He wakes up from that dream again with his sister waking him up, making him go to school. On his way there, he meets a girl who trips over a rock and lands right on him, passed out. He brings her to the school's nurse's office. In class, a new girl comes running in shouting Shinichiro's name. She looks awfully familiar to the girl Shinichiro brought in earlier. The class leader knocks the new girl out for being to rambunctious and Shinichiro brings her to the nurse's office too and finds that there are actually two girls who look very similar. When they come around, they get officially introduced. The girl he saved in the morning is named Sawatari Arisa, the loud one is named Sawatari Alyssa. While their names are identical in pronunciation, they have no apparent relation. Shinichiro looks at Arisa and thinks she's the girl from his past as she's very bashful and shy. However, Alyssa wears the cat-ears that he remembers and she also remembers him. When he goes home, he finds out that his uncle, who he lives with, has taken the two girls into his house.