The heroine is a mixed race, Sanada's sister, the hero Tezuka, his grandfather is the godfather, and his mother is from the Sanada family.
The heroine and the hero were called by the wrong number from the heroine. This mistake turned them from strangers into friends. They shared their emotions with each other regardless of spring, summer, autumn, winter, happiness, anger, crying, etc. No one would have thought that a phone call would make them develop feelings for each other...