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"...
Come to think of it, you've been a bit 
of a worrywart ever since you were little.
Ahora que lo pienso, 
desde pequeño te preocupabas por todo.
Ahora que lo pienso, 
desde pequeño te preocupabas por todo.
A pensarci, fin da piccolo hai sempre avuto
un carattere un po' pessimista.
Pensando bem, desde pequeno,
você sempre esteve condenado a sofrer.