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"...
That's why, of all the people I've ever met,
Deswegen macht mir
Haru von allen Leuten,
Por eso, de todos los que conozco,
Por eso, de todos los que 
he conocido hasta ahora,
Parmi les gens 
que j’ai rencontrés jusqu’à maintenant,
Per questo, tra tutte le persone
che ho conosciuto,
É por isso que, de todas 
 as pessoas que já conheci,