After her apartment is destroyed by fire, Koharu moves into the home of her boyfriend, Arata, and his family. However, she quickly falls prey to Arata's father, who blackmails her into a secret sexual relationship using compromising footage. Just as Koharu begins to spiral into a cycle of guilt and pleasure, Arata's brother discovers the truth and decides to join in on her exploitation.
Koharu struggles with her guilt as she continues to be abused by A-kun's father and brother. After being subjected to a powerful aphrodisiac and a cruel challenge, her resolve completely shatters, and she embraces her new role as the household's communal plaything. Ultimately, Koharu betrays A-kun, casting him out of the house to fully surrender to the hedonistic desires of the "beasts," leaving A-kun to watch her degradation from afar.