
When her older sister’s son is kidnapped, Kotomi, who is determined to help her and her brother-in-law, goes against the kidnapper’s demand and notifies the police. The detective (Keisuke) in charge of the case surrounds the ransom drop-off location to make the arrest, but the kidnapper never shows up and the son is never seen again…Two years later, Kotomi gets married to a man named Makoto. Her sister and her husband pay Kotomi a visit to throw a party and celebrate this wonderful occasion. That night, the two sisters, who had grown apart since the kidnap incident, slept alone together. Kotomi is awakened by an alarm clock, only to find out that she is not in her room, but in a ruined deserted factory with only her underwear on and limbs tied. Next to her lies her sister tied up in the same fashion. In front of the two sisters calling for help appears a man wearing an ominous mask. The man’s deranged game begins as he repetitively violates the sisters.
Cheating Notes

(spoilers)
Note: read the movie description above first.
More than halfway through the movie, it is revealed that the ones who kidnapped Kotomi and her husband are her older sister and her brother-in-law as revenge for their kidnapped son who was never returned to them because Kotomi informed the police. They just pretended initially that Kotomi's older sister was kidnapped too, along with Kotomi. The one wearing a mask who rapes Kotomi several times during her captivity is her brother-in-law (but at one point it's Kotomi's sister who was wearing it to mislead her).
At 51:30, Kotomi, chained to a wall, witnesses her older sister ride her (Kotomi's) husband in front of her. She doesn't recognize that it's her husband initially because a cellophane bag is covering his head. Kotomi's brother-in-law, still wearing his mask and pushing the wheelchair of another prisoner, comes in and sees his wife riding Kotomi's husband. He doesn't get mad but instead, he takes his turn in fucking his wife on top of the body of Kotomi's husband. He does her doggy style initially and switches to other positions later.
srt please