Prefacing this novel, Ian McEwan quotes from the Children Act (1989) from which it takes its title:

"When a court determines any question with respect to ... the upbringing of a child ... the child's welfare shall be the court's paramount consideration."

What follows is a meticulous yet profoundly emotional example of the ways in which the demands of law can be in direct conflict with those to whom it is applied. It also asks to what extent one can ever determine the best outcome for a child.

The situation under jurisdiction is that of a boy a few months shy of his 18th birthday. Adam is suffering from leukaemia and needs a blood transfusion, without which he will either die horribly or risk being permanently disabled. As a devout Jehovah's Witness he, and his parents, are refusing the treatment.

With only a few days in which to make a ruling, the case is given to High Court judge Fiona Maye, an expert in family law. Visiting Adam in hospital, she is impressed by his articulacy and intelligence. She is also moved on a more personal level. Even so, she must set aside emotion, and make a decision, all too aware she represents a secular institution whose wholly rational basis is at the polar extreme from that of the believers whose fate she is determining.

McEwan describes Fiona with such intensity, he could be trying to evoke a character from a previous age. In fact, at times her world seems far removed from life as most of us know it. Gray's Inn Square, where she and her husband Jack live, is a haven of elegance and culture, as are their lives: he an academic, she a gifted amateur pianist, their Saturday mornings spent sipping fine coffee and listening to Poulenc, their evenings spent at soirees and concerts.

All is not well with them, however, as the opening page bluntly shows. Thus, at the same time as calibrating fine legal arguments that affect lives such as Adam's, Fiona must also deal with the fault lines in her 30-year marriage, whose barrenness is more the result of expediency than any calculation.

With the thoroughness of one newly versed in his subject, a feature increasingly evident in his recent novels, McEwan portrays the workings of a court, or a judge's day, and the sort of society in which a judge mixes. Long before he introduces Adam, The Children Act has described other high-profile cases that have helped make Fiona's name. It is all fascinating, but a little chilly, despite the life and death stakes with which she is faced.

With a plot revolving around the conflict between secular and faith perspectives, McEwan allows Fiona to acknowledge that "she belonged to the law as some women had once been brides of Christ". As the novel's narrative tension rises, she views her own position with conscious dispassion: "Religions, moral systems, her own included, were like peaks in a dense mountain range seen from a great distance, none obviously higher, more important, truer than another. What was to judge?"

Beneath the drama of Adam's predicament, McEwan threads a bitter undercurrent of distaste for selfish greed: "She thought in idle moments that she could send down all those parties wanting, at the expense of their children, a younger wife, a richer or less boring husband, a different suburb, fresh sex, fresh love, a new world view, a nice new start before it was too late. Mere pursuit of pleasure. Moral kitsch." In part this is an echo of her own marital discord, but it is also the wearying effect upon her of the cases she encounters every day.

The cool personality and brilliant mind that distinguish Fiona are replaced, briefly, with a blaze of passion and irrationality, setting her, and the novel, alight. But while one can admire McEwan's flawless execution of a clever idea, and appreciate his skill in catching the mood of our morally complex times, The Children Act feels like an exercise in ethical quandaries, a sophisticated fictional response to a fiendish legal dilemma. Perhaps the research is too evident, or maybe his heroine's low-key nature infects the whole. Whatever the reason, a sense of claustrophobia envelops this short novel as if it, like the cases it discusses, is destined for the legal annals, rather than for the rough and tumble of the fiction shelf.