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"...
I haven't even lit fireworks with you yet,
und wir müssen noch ein Feuerwerk machen
und fischen gehen.
Todavía no encendí
 fuegos artificiales contigo,
Todavía no encendí
 fuegos artificiales contigo,
Non abbiamo ancora fatto i fuochi d'artificio
 e non ti ho nemmeno portato a pesca.
ainda não soltei fogos com você,