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"...
Well, you'll just have to do 
your best to make him human.
Ach, man muss sich nur richtig anstrengen,
dann wird daraus auch ein Mensch!
Bueno, tendrás que esforzarte 
para hacerlo más humano.
Bueno, tendrás que esforzarte 
para hacerlo más humano.
Si on y met du nôtre,
Beh, dovrai fare del tuo meglio
per farlo diventare un essere umano.
Bem, boa sorte para transformar
aquilo lá em uma pessoa.