Haru Kaidou returns to Canada after hearing about his mother's health, only to discover he's supposed to care for his adoptive brother Ren Kaidou, an antisocial boy who prefers dogs over people. Initially, Haru struggles to connect with Ren due to his brother's distrustful nature. Eventually, they form a bond, and Haru makes a promise to Ren to live together in Japan after he graduates from high school. However, an accident erases Haru's memories of their summer together, including the promise.