Books: A fatal clash en route from Temple to Westway

In English courts, barristers do not have to believe in their clients' innocence as long as they do not know for certain that they're guilty. This principle is intended to ensure that even the most reprehensible criminal has a proper defence, but the conflict and confusion it creates are at the heart of P D James's new novel. Garry Ashe, a psychopathic young man accused of killing his aunt, gets off with the help of a brilliant barrister whose job, as she sees it, is to make sure he gets the benefit of reasonable doubt.

What Venetia Aldridge does not expect is that the young man in question will then become engaged to her rebellious daughter. The murder of which he has just been acquitted was particularly brutal, with the dead woman's body found on a bloodstained sofa in the squalid council house they shared near the Westway. At the trial, Venetia made no attempt to deny that her client regularly photographed the victim having sex with a series of boyfriends. Yet, even though a neighbour had identified Ashe as the man she glimpsed leaving the scene of the crime, Venetia manages to create sufficient doubt about identification for him to walk free.

Free, but not out of her life. The case is just another milestone in Venetia's career until 18-year-old Octavia confronts her with the news that she is in love with Ashe. Venetia immediately smells a rat, especially when she hears Octavia's account of how they met - supposedly by chance, after an accident outside the house where the two women uneasily co-exist. When Venetia reminds Octavia of the circumstances of the murder, and the likelihood that aunt and nephew were lovers, her decision to defend Ashe is thrown back in her face. "You told the court that he didn't do it," cries Octavia. Her mother's response, outlining the difference between innocence and reasonable doubt, falls on deaf ears. But the reader knows that the showdown to which all this seems to lead will never take place, for James's bold opening has established that Venetia herself will soon be dead, murdered four weeks after her triumph. It emerges, when Commander Adam Dalgliesh begins to investigate, that she was widely admired but not liked.

This is a classic English detective novel and a circle of suspects is soon identified, including Venetia's colleagues in Pawlet Court, her clients and characters from her sparse private life. Garry Ashe features in two categories and logic identifies him as a prime suspect - except that he has an unbreakable alibi.

A Certain Justice is a very traditional puzzle with a setting - the dark staircases and quiet courtyards of the Middle Temple - whose traditions hark back several centuries. Its literary antecedents belong in the Golden Age of the 1930s, yet James brings to it a sly modernity, both unexpected and shocking.

Neither a lament for a vanishing world nor an outright condemnation of what has taken its place, this beautifully-written book is a complex exposition of a conflict between values which leaves a trail of corpses. The synthesis of gripping narrative and bleak moral vision offers so much more than we expect from the detective story that it is hard not to marvel at its scale and ambition. Unlike so many crime writers, James still has the power to move, fascinate and astonish.