Based on the erotic game by Terios.

Chihaya & friends come to the beach for summer camp. There they meet their classmate Nami Minami. She likes Natsuki, but doesn't have the courage to talk to him. As the darkness inside Nami grows, she finds an old mansion where the "Netherworld Tree" awaits her. It was the start of the incident that would rock the boat?

(Source: AnimeDB)