唐和他的妻子弗朗西斯一直在婚姻中挣扎,弗朗西斯有一个患有精神病的妹妹凯伦和他们住在一起,但凯伦的存在对他们的浪漫造成影响,因此两夫妇一直在考虑让她接受庇护。凯伦无意中听到他们的计划后她逃跑了,躲在一个偏远海滩上的空房子里,在那里她经历了残酷的折磨…… well-to-do couple, Don and Frances has been struggling in their marriage. Frances's mentally ill sister, Karen, who lives with them, is taking a toll on their romance and as such. They bicker about what to do with the grown woman - who's gone mute since staying with them - and has even turned to their psychologist chum Otis and his shrill, unsatisfied wife Estelle for advice. Upon hearing they have been considering having her committed to an asylum, Karen flees to an abandoned beach house on Fire Island, where she endures a series of gnarly sexual encounters that push the voices in her head to finally lash out through her murderous hand.