During the First World War, mud was where men died. It was also, occasionally, where they were reborn. There is a scene in Ford Madox Ford's novel Parade's End in which Tietjens, the hero, pulls free a young wounded lance corporal who is stuck up to his chest in mud. Tietjens, writes Ford, "felt tender, like a mother". Pat Barker has had a long-running imaginative relationship with mud.

It clogs, kills and terrifies throughout her First World War Regeneration trilogy. In the opening sequence of the film version (1997), the camera pans across a silent shire of mud in which dead and half-dead men are embedded. And mud -riverside, urban mud -also begins her new novel, Border Crossing, in an episode that recalls Tietjens's deliverance of the young lance corporal. Out walking one morning, Tom Seymour, a child psychologist, sees a boy leap into the frigid and tidal waters of the Tyne. He dives in, and lugs the boy to the mudbanks that border the river. Near "the groyne of the bank", Tom pulls the boy to safety. "Black and glistening, he lay there, a creature formed, apparently, of mud . . . . The boy looked like a baby: purple face, wet hair, that drowned look of the new born."

The boy whom Tom so symbolically delivers from death turns out to be Danny Miller, a former patient of Tom's, convicted at the age of ten for killing and then mutilating a seventy-eight-year-old woman. After thirteen years of imprisonment and rehabilitation, Danny is now out on parole, possessed for his own protection of a new identity, but not necessarily of a new personality. His suicide bid, it transpires, was staged: a ruse to get back in contact with Tom, although it is unclear whether he is seeking help or revenge, for Tom was the psychologist who declared Danny fit to stand trial for murder. The relationship between Tom and Danny forms the centrepiece of the novel.

Barker has long been interested in crimes that break taboos, that cross moral or psychological borders. Blow Your House Down (1984) was based on the Peter Sutcliffe murders. Another World (1999) was sidelit by the James Bulger killing, and featured several incidents of child-on-child violence. Here, the crime is one which violates the limits both of generation and gender. There are several aspects of these anathema crimes which particularly interest Barker, and Border Crossing seems to have been constructed principally to permit her to explore these aspects; much of the novel consists of extended dialogues between Tom and Danny, in which the nature of evil is debated and dissected. Barker wants to know if we can understand, explain, or even describe these crimes using our conventional diagnostic and aetiological models. Are they, as it were, within our imaginative ken? How does the individual memory shape and reshape the contours of the act itself? And -the issue which concerns her above all in this novel, as it did in the Regeneration trilogy -can a person be psychologically reborn after a major trauma? Is regeneration (in this case of the criminal and not of the shell-shock victim) possible?

The obvious parallel which Barker expects us to draw is with the two Bulger killers, who are soon to be released on parole with fresh identities, after eight years in secure units in the North of England. Their imminent emancipation has focused the debate on the rehabilitation of extreme criminals.

Although it is difficult to pin Barker herself down to a position, she seems eventually to imply that for people - criminals -to be able to change, the expectation that such change is possible must first be ingrained in the criminal justice system, and in society.

This is a brave novel, but it is not a great novel. A chapter set in a creative-writing retreat reads as though it was written in one. The bit-part characters are unconvincing, and seem to be marched on as placard-bearers in the debate. The solicitor who represented Danny, for instance, appears briefly to declare him a "precocious little killer", and to assure Tom that "some people act out of a disinterested love of destruction", before disappearing back to his office. There is often a lapse from the picture to the diagram; one feels that Barker is simply voicing the various sides of a moral- philosophical conundrum (a problem with much of Iris Murdoch's fiction). Though the discussions between Tom and Danny demonstrate Barker's magnificent feel for dialogue and idiom, the segments connecting the dialogues are generally unmemorable, written in a prose that is flattened almost to the point of dullness. The text, one feels, could be converted without much loss or damage into a 100-page play script. And, of course, we take the measure of Barker's novels against the now canonical achievement of the Regeneration trilogy, in comparison with which Border Crossing feels definitely slender.

Despite these shortcomings, however, one leaves this novel with the sense that a very important issue has been grappled with. "Is there something wrong with modern England?", George Walden asked, as he back- handedly awarded Pat Barker the 1995 Booker Prize for The Ghost Road. "Why do our novelists shy away from us? Do we give off a bad smell, like old vegetation? The flight from the present is becoming a general phenomenon. If the past is another country, then we are facing a sort of mass emigration. Nostalgia is becoming our heavy industry."

Barker later rejected Walden's indictments, quite rightly pointing out that retrospection does not automatically default into nostalgia, and that the present must often be read through the past. The First World War in particular, she observed, "never becomes part of the past . . . we stop remembering at our peril".

Nevertheless, Barker has modernized herself dramatically since 1995. Her fictional environment, for five years the killing fields of France, has become the modern home front -embattled NHS wards, urban violence in sink housing estates and on inner-city streets, the trench warfare of marriages -and in her last two novels she has squared up to some of the most intimidating of latter-day social issues. She refuses to be horrified by these issues, for to be horrified is to avert one's gaze. She prefers, as Thomas Hardy put it, to exact a full look at the worst. This is to be applauded.

BORDER CROSSING by Pat Barker

224pp, Viking, £16.99

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