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"...
Oh, man, I think I'm in trouble now...
Irgendwie weiß ich gerade
gar nicht, wie mir geschieht …
Vaya, creo que estoy en problemas…
Vaya, creo que estoy en problemas...
Quel est ce sentiment
qui m’envahit ?
Oh, ragazzi, ora sì che sono nei guai...
Acho que... A minha 
 sanidade foi comprometida...