Without revealing too much about the plot (though anyone who's read the Greeks will see the nature of the novel's climax coming), the story concerns four children, ranging in age from the youngest, Tom who is about six or seven, to the eldest, Julie, who is a beautiful seventeen year old. Their parents die in quick succession, and the story concerns what happens after they are left to their own devices, isolated and with no supervision.
The brutally offhand tone with which McEwan tells this story, so jarring in light of his later work, is actually appropriate for the kind of tale he seems to have intended. The device that allows him to reach this tone is his narrator - the eldest brother, fifteen year old Jack. McEwan captures the naturally obsessive self-concern of the teen-ager (yes, I'm generalizing), and in this way executes a chilling series of events without making the reader feel that the children are actively monstrous. They are simply young, and the young can be quite monstrous at times, and at other times, nothing but pathos.
I've read some reviews of this book in which the reviewers were either profoundly disgusted or terrifically disturbed by The Cement Garden. I suppose I can see why, but I would say that this makes it a successful novel. Disturbing things occur, but the story drives forward and remains incredibly readable. It's the offhand delivery of disturbing events that make the novel thought-provoking and worth the read. You'll either like it or you won't, but you will not be ambiguous about it, and that, I would say, is quite a compliment to Mr. McEwan.