Having come to Canada upon hearing the fake news that his biological mother Haruko is in critical condition, high school student Kaidou Haru meets a young boy named Ren. "You have a new brother. Take care of him." So instructed by his mother, Haru is forced to spend the summer with Ren. Every day is a battle with Ren, who refuses to listen to anybody. However, as Ren grows to like Haru little by little and Haru comes to adore Ren as well, they make a certain "promise"...
Be glad, Haru. You have 
a new little brother now.
Freu dich, Haru. Du hast jetzt
einen neuen kleinen Bruder.
Alégrate, Haru. 
Tienes un nuevo hermano menor.
Alégrate, Haru. Tienes un 
nuevo hermano pequeño.
Réjouis-toi, Haru.
Congratulazioni, Haru.
Adesso hai un nuovo fratellino.
Alegre-se, Haru. Você acabou de
ganhar um novo irmãozinho.