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"...
He was so antisocial, they didn't 
know what to do with him.
Er war ungesellig und keiner wusste,
was man machen sollte.
Era tan antisocial que no 
sabían qué hacer con él.
Era tan antisocial que no 
sabían qué hacer con él.
Ils étaient dépassés
par cet enfant insociable.
Non sapevano cosa fare
 con un bambino così asociale.
Eles estavam sem saber o que fazer
com uma criança tão antissocial.