Haru Kaido goes to visit Canada and spend his summer vacation in the woodland house of his strict, strong-willed mother Haruko who purposely fools her son into believing that she's on "the verge of death" only to burden him with the responsibility of taking care of her newly adopted "puppy". That "puppy", in actuality, is a small, rash, anti-social little boy named Ren who doesn't trust anyone and would prefer hanging out with Haruko's dogs. Haruko orders Haru to "civilize" Ren until the end of summer. As the young man struggles in doing everything he can to help Ren, their relationship gradually improves; so far so that when Haru graduated high school, he planned on taking Ren to Japan for them to live together with his twin half-brothers.
Unfortunately, an accident occurs that causes the death of Haru's father and stepmother, leaving him the only survivor. The accident causes him to lose his memories of that summer.
Five years later, Ren comes to Tokyo, Japan and claims Haru as his new "brother". Will Haru adjust to his new role in life and keep that promise to Ren? Or will he deny Ren as his brother?
