Based on the erotic simulation game by Dieselmine.

You're spending a boring summer vacation cooped up in your postage stamp-sized apartment, when you receive a call from your relative's mom. It seems she and her husband are going on a trip, so they want you to come back to the countryside, and look after your old childhood friend Hazuki for a while. Well, it certainly beats melting here in the city, so you decide to accept the offer. So you start preparing for the journey...

(Source: DLsite English)