Claris Manning is a middle-aged, wheelchair-bound, unlike able, hopeless alcoholic who lives in a large suburban house with her youngest daughter Carol, a local model. Also residing there is Claris' manservant Mortimer, whom has a black eye patch over his right eye from a time back where Claris deliberately stabbed him in the eye a few years back. Also residing in the house are the servants Peter and Jessica. Carol tells her mother that she invited her siblings over for Christmas Eve dinner the following day. Claris throws another tantrum for she dislikes her children as well as dislikes the whole world. Peter and Jessica secretly talk about plotting to steal all of Claris' money and skip town since the misanthropic woman treats them badly. Peter and Jessica agree to their plan by having sex in the kitchen.
Carol, having put off her social life so she can take care of her mother despite the fact that Claris treats her badly, spends her lonely time reading mens muscle magazines and masturbating her bedroom to the photos. When Claris has another seizure, Mortimer is forced to call upon Dr. Kram and his nurse, Miss Bundy, to come over and administrate more medication. Yet afterwards, Claris does not thank them and continues drinking and smoking despite her worsening health.
Having taken care of Claris, Dr. Kram returns to his office where Drew, one of Carol's brothers, is there and asks to help perform an abortion on his girlfriend, which the doctor agrees for a $1,500 pearl necklace that Drew had stolen.
At the Manning household, 17-year-old Buster, the youngest sibling, arrives first having taken time off from the miliary academy where he resides and is greeted by Carol whom they are apparently the most closest. Next, Michael and his wife, Susan, arrive where it is hinted that Carol and Michael have had an incestuous relationship in the past. Also arriving is Carol's older brother Matthew, a crackpot clergyman, and Carol's older sister Margaret, whom is always at odds with everyone. The dinner proceeds very uncomfortably as Claris does not stop yelling and insulting everyone.
Afterwards, Barbara comes into Michael's room where it is revealed that they are having an affair. When Barbara goes to take a bath, an unseen person knocks a plugged-in radio into the bathtub, electrocuting her. Susan arrives where she and Michael get into an argument over Susan over their pasts and of both of their unfaithfulness. Susan runs out and while contemplating suicide, a unseen person stabs her with the knife she is holding.
The next day, Claris is indifferent when she learns from Mortimer about the suspected "suicides" of Susan and Barbara. The grouchy old lady suspects that the deaths were not accidental or suicides... but murders and thinks that one of her children committed the deed and she may be next.
A little later, Buster comes out to his mother about having homosexual feelings, she naturally puts him down and insults him with homophobic slurs. When Claris tells Buster that she intends to sever not only him and everyone out of her will, the distraught Buster stumbles away and into the nearby woods where he slashes his wrists with a broken beer bottle and stumbles away to bleed to death.
Back at the house, Peter and Jessica debate on how to make their move now to steal Claris' money and run off, when they leave the kitchen, the killer poisons both of their drinks and upon returning to the kitchen, they have a drink and both of them fall dead.
Matthew, in his room, is attacked by the unseen killer who wraps a rope around his neck and hangs him from an overhead ceiling fan.
Meanwhile, Margaret's husband, Jonathan, a tough street youth, arrives where the two of them engage in sex. When Margaret goes into the bathroom to take a bath, the unseen killer throws acid on her face, horribly scalding it, and she dies an agonizing death. Jonathan runs in, but gets beaten to death by the killer before he can do anything.
Carol goes into Michael's room where he confides in her that Susan had deliberately gotten pregnant to trap in him marriage, and expresses doubt that she really did kill herself. Carol and Michael begin to engage in sex when Michael stops her, telling her that this is wrong between them. Carol then lets it slip that she murdered Susan, Barbara, and everyone else for all these years of living with her mother which drove her crazy with vengeance. She even killed Peter and Jessica since they were planning to kill Claris for her money and wanted them out of the way so she could do it herself. Michael throws Carol out of his bedroom in which Carol, stark nude, goes up to her mother's room and expresses to have killed everyone so she can be at peace. Carol finally takes her revenge when she wheels her mother to the stairs and throws her, wheelchair and all, down the staircase in which Claris dies almost immediately. Michael comes upon the scene and attacks and strangles Carol for what she's done. Mortimer walks in having observed the events. Relived that everything is over, Mortimer makes a phone call to the police while Michael just looks on.