There are two very close siblings.
Their free-spirited parents work overseas, but the family has large savings.
They live a comfortable life, with no inconveniences.
One day, the protagonist's father, who lives overseas, calls to announce that he has adopted a new sister.
Because the sister (stepsister) has returned to Japan alone, the protagonist is being given a serious request.
The two want to warm up to their unexpected stepsister, but she is traumatized and rejects others...
As the older brother, the protagonist struggles every day to become a true family.