The three begin to talk, remembering their mother. A thudding noise is heard below, and their sister Sue comes and informs them that Derek is smashing up the concrete coffin. He remarks that he has seen it all and calls them 'sick.” When he leaves, Jack and Julie begin to have sex. While talking, Jack and his older sister become more and more intimate with each other. Julie enters and, seemingly unsurprised by her younger brother's nakedness, jokes that 'it is big.” Jack and Julie sit on the bed while Tom sleeps, and Julie takes off her clothes. Jack climbs into Tom's cot and begins to talk to him about their parents. Jack enters naked into Julie's bedroom, apparently absent-mindedly.

Eventually, Tom tells Jack that Derek has told him he believes their mother is in the cellar. Derek then helps to re-seal the cement which their mother is hidden in. When a smell begins to emanate from the cellar, the children tell him their dead dog, Cosmo, is encased in the cement. Derek gets a hint that something is hidden in their cellar, and becomes more and more interested while the children attempt to hide it from him. When Julie begins to date a young man called Derek, aged 23, and invites him to their house, Jack feels jealous and shows hostility towards him. Sexual tension between Jack and his older sister, Julie, becomes increasingly obvious as they take over the roles of 'mother' and 'father' in the house, which is gradually deteriorating into squalor. Jack then mentions how he longs to do the same to his older sister but it is not allowed. Jack describes how, when they were younger, he and Julie would play doctor with their younger sister, although he is aware that their version of the game occasionally broke boundaries. The narrator is Jack (14 at the start of the book, becoming 15 later), and his siblings are Julie (17), Sue (13), and Tom (6). The children then attempt to live on their own.

In order to avoid being taken into foster care, the children hide their mother's death from the outside world by encasing her corpse in cement in the cellar. Soon after, the children's mother also dies. In The Cement Garden, the father of four children dies.

The cement garden summary