During his childhood, the protagonist homestayed at a certain family in USA. There, he met a girl around his age whom he quickly became friends with.

Several years later, he was informed by his parents, who are in the USA, that the girl he used to be friends with will be coming to Japan as a foreign student, and she will be homestaying at his house.

"It's been ages!"

And so, cohabiting life with the aggressive foreign student, blond hair childhood friend begins.

[From the official site]