When you were younger, there was a girl in your extended family that you played together with just like brother and sister... However, after your family moved to Tokyo, the two of you naturally became distant.
Several years have passed since then and your family has decided to head back to your hometown for summer vacation. For the first time in a while, you receive a message from the girl...

"I want you to instruct me about sex..." she has written.

...and along with that message, she has sent an erotic selfie of herself.
Thus began your secret summer vacation together.

[From DLsite]