In a rural hot spring town, a grandmother lives with two children, an older boy and a younger girl.
As the boy has no relatives, the grandmother tells him "I have a favour to ask, grow up so that you can make our lives comfortable".
But it was only an idea, while life continued.

After that, several years passed...

After passing an exam, he entered a new school.
Having achieved the highest grades of anyone, the grandmother was overjoyed, but she suddenly died.

He hadn't finished her favour.

About a year later, it's the start of the summer holidays. A "lying girl" appears, and his surroundings start to become noisy...