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"...
Why do those two love you so much?
Warum mögen nur die
beiden dich so sehr?
¿Por qué te quieren tanto?
¿Por qué te quieren tanto?
Pourquoi ces deux-là
t’aiment autant ?
Perché quei due ti vogliono così bene?
Por que só esses dois 
 são tão dóceis com você?