Hikari Shinomiya's father left when she was in elementary school and she lives with her mother.
While she tries to act brave so as not to worry her mother, her loneliness disappears when Noah, the half-Japanese boy who lives next door, promises to become her older brother
. It was.
Hikari, who has fallen in love with Noah, who is always with her at school and is closer than necessary, is confused about how he treats Noah as her younger sister...

After entering college, Hikari's mother and Noah's father, who are both single parents, officially get married!
Hikari is happy, but is shocked to find out that she and Noah have become "brother and sister" in both name and reality.
However, Noah's skinship becomes more intense, and he finally kisses her on the lips, and his tongue enters her...?