Demon's Room (1982)

Young woman Yoshiko (Reiko Nakamura) gets a call from a man, Fuji (Johnny Okura) claiming to be from her husband's company. He tells her that he will come to pick her up on behalf of her husband, Hiroyuki (Masami Horiuchi) who is the scion of a hotel magnate. The kidnapper is Hiroyuki's father's secret love-child. He takes her to a hotel, and proceeds to rape her. He calls her husband but doesn't request a ransom. He's just bent on humiliating her to submission to be his sex slave, but she begins to fall in love with him even as Hiroyuki is finally deciding to pay the ransom to free her.