One day, a big brother who blindly loves his little sister
eavesdrops what she talks on the phone.

She talks about future plans with a man and he hears that
they say something like "If we go to the same school...".

When it is implied that she has a boyfriend, he thinks in such a way,
in despair, that the boyfriend must have bad influences over her so
he confines her in the house in an attempt to isolate her from the guy.

Exploiting the situation where he and she are left alone in the house
during the summer vacation, he starts disciplining her to reforge their siblinghood.

[From DLsite English]