Based on the erotic game by POISON.

Kyoji runs a hot spring resort inn, but he has a secret side to him. He is a sexual demon who seduces the female guests who stay at the inn and distributes the sex videos. This time, his target, the Hidaka family, is a "priceless customer" with a beautiful mother and two high school daughters. He sneaks past the eyes of the troublesome men who have followed him, and threatens them to get them. To carry out his plan, he also calls in his despicable younger brother and his younger brother's girlfriend to help him. And so begins the fun of a hellish vacation.

(Source: Parabook, translated and edited)