The film begins with Jack, a narcisstic 15-year old boy, helping his father, a smoker with a persistent cough, unload big bags of cement to resurface the garden path of their post-war era prefab house. Despite being told to come straight home from school to help with the work the next day, Jack stops by the remains of a torn-down prefab to smoke a cigarette and read a pornographic magazine he has hidden there. When he comes home, he excuses himself to go to the toilet, where he proceeds to masturbate. As he is doing so, his father collapses from a heart attack and dies.
A few months after the tragedy, Jack's mother and his siblings - 17-year old Julie, 13-year old Sue and 6-year old Tom, are still mourning, and Jack has become apathetic to the feelings of his family, while also neglecting to wash. As suddenly as their father's death, their mother is taken ill and becomes bedridden and frail, forcing Jack and Julie to take control of the house. Jack's selfishness soon becomes a conflict between him and his older sister. When he attempts to remedy the tension between them by tickling her with bee-suit gloves, he accidentally causes her to orgasm, although he attempts to cover this up from his siblings. Shortly after this, the family celebrates Jack's 16th birthday in their mother's bedroom. He is reluctant to contribute to the small party by singing, but when Julie does a handstand, Jack stares at her underwear, before engaging in a mini love scene with her by singing Greensleeves. Although the rest of the family assume they are having fun, Julie recognises what he is doing, and plays along. This, along with another incident wherein Julie, while sunbathing in the garden, persuaded Jack to rub suncream into her back, intensifies what was initially a small affection for her into an incestuous crush. This makes things more complicated for them as they are forced to take on parental roles in the house with their mother bedridden. Jack soon finds himself escorting Sue and Tom to and from school, and defending Tom from a school bully, while Julie takes up the cooking and cleaning duties.
One day, while cleaning up his mother's room, Jack's mother informs him that her illness has become worse, and that she will have to go to hospital for a couple of months. She tells him and Julie not to tell anyone about her absence, or else they will be forced into foster care and their home torn down, and informs him that she has set up a savings account for them to take care of themselves for a while. Shortly afterwards, the mother dies of her illness. Grief-stricken and with no-one else to go to, the siblings spend a miserable night together consoling each other in their living room. Remembering their last promise to their mother, Jack and Julie secretly resolve to hide her body. That night, they take the remaining bags of cement that Jack's father had bought the day before he died, create a mixture of cement and entomb their mother's body in a cement sarcophagus.
As time passes following their mother's death, Jack slowly matures, although he stops washing altogether, while Sue becomes more introverted, routinely distancing herself from her siblings and confiding only in her diary, and Julie forms a relationship with a much older man named Derek, whom Jack starts to view with jealousy and hostility. Tom, meanwhile, takes up cross-dressing, and attempts to remodel himself as a girl, something that irritates Jack, but delights his sisters. Eventually, Tom starts to relapse into a baby's state of mind, and takes to sleeping in Julie's room in a crib and drinking from a bottle. The mental states of the other siblings also gradually decline as time passes with no adult supervision. Derek, meanwhile, becomes more and more interested in what they are hiding in their basement. After a smell emanating from the basement draws him downstairs, Jack steps in and lies that the body contains a dead dog, subtly referencing their mother in the process.
In a climatic final scene, Jack falls asleep naked on his mother's bed. He wakes up to the sound of Tom crying, and goes into his sister's room, and joins his younger brother (who is also naked, due to the heat) in the crib. Tom tells Jack that Derek has told him that the sarcophagus actually contains their mother, and Jack realises that Julie has told Derek the truth. Jack tells Tom fairytale stories to coax him back to sleep, and soon falls asleep himself. He is awoken by Julie, who is both delighted and amused at what she has seen. She sits Jack on her bed, and explains to him that she and Derek are not physically intimate, and that he is an immature adult who lives with his mother. As their conversation becomes more intimate, Julie takes off her own clothes, to join Jack in his nudity, and the two cuddle on the bed, while contemplating the future now that too many people know their secret. Jack predicts that they will be taken into foster care, and their house will be torn down like the other prefabs in the area, pondering "one day, someone will come rooting round. All they will find will be a few broken bricks in the long grass." At this moment, Derek walks in, and is horrified to see the two siblings cuddling naked on the bed. He expresses disgust and horror at what the two have become in burying their mother in cement, and their incest. Unperturbed, Jack and Julie lie back down on the bed, and converse further. Jack declares that their situation "Seems natural to me." Smiling, Julie responds "Me too," and the two begin to have sex, as the sound of Derek smashing up the cement tomb containing their mother reverberates around the house.
The movie ends with Jack and Julie, post-coitus, asleep in bed, as police lights illuminate the room.
