Five years prior to starting high school, Shinobu Omiya left Japan and visited England to participate in a homestay. While in England, she made very good friends with the daughter of the family she was staying with, Alice Cartelet. Despite not speaking the same language, the two grew very close and shed many tears when it was time to say good bye to each other.
Now in high school, Shinobu is perplexed when she receives an air mail letter written in a language that she can't understand. It turns out that the letter was from Alice, and was telling her that she would be coming to Japan to participate in a homestay of her own. Not only that, but she would be living with Shinobu and going to the same high school as her!
From this point forward, Kiniro Mosaic follows the daily lives of Shinobu, Alice and their friends Aya, Yoko and Karen.