When Hikari Shinomiya was in elementary school, her father left her and she lived with her mother. While she tried to act strong so as not to worry her mother, her loneliness was eased when Noah, a half-Japanese boy living next door, promised to be her older brother. Hikari was always with Noah at school and was closer than necessary, but she was unsure of the fact that he treated her like a little sister.

When she became a university student, Hikari's single-parent mother and Noah's father officially got married! Hikari was happy, but she was shocked that she and Noah became "brother and sister" in both name and reality. However, Noah's skinship became more intense, and he finally kissed her on the lips and even used his tongue...