If People Who Eat Darkness were a novel, the reviewer would sketch the outline of the plot without giving away any essential twists, leaving the suspense intact. But this is a true story that has been in the public domain for many years. Broadly speaking, there are few surprises  though to anyone suffering from usual levels of media amnesia, it is full of heart-stopping revelations. This reviewer was averagely conscious and averagely ignorant of the story of a young woman who had disappeared in Tokyo  wasn't her body found buried in a bathtub?  but beyond that everything, including her name, was hazy. Others may have remembered the case more clearly but, as journalist Richard Lloyd Parry observes near the end of his devastating account, "people became so confused between the British girl buried in the cave and the British girl in the bathtub that they came to seem like a single incident." This is the story of the girl in the cave.

If it had been about the girls buried in the basement I would not have gone near it. Gordon Burn apparently wrote an excellent book about the murderer Fred West but there is no chance of my ever opening it. Whenever I saw the words "Crossbow Cannibal" in headlines a few months back I turned the page quickly. But People Who Eat Darkness would also be a story about Tokyo, and the expectation  here again I was conforming to type  of learning something "about Japan and Japanese people as a whole" was irresistible. Parry has a knack of tacitly cross-examining his readers in this way, not implicating them exactly, but immersing them in a darkness that thickens as facts come to light.

The basic facts are these. In 2000, 21-year-old Lucie Blackman from Sevenoaks went to Tokyo where she found work as a hostess at a club in the Roppongi district. This did not mean prostitution or topless dancing. It meant soaking up the talk  frequently flirtatious, mainly just boring  of drunken businessmen so that they would buy more booze. Her role was precisely defined by the codes of Japanese life but  and this is the first of many instances where uncertainty seeps into what seems rigidly unambiguous  to make more money, hostesses were obliged to see some of their clients beyond the dark safety of the club. It was on an outing like this, less than three months after arriving in Tokyo, that Lucie disappeared.

Police were alerted and began an investigation whose sluggishness was partly and tacitly due to the fact that western women in this line of work are regarded as being on the fringes of the sex industry, or the kind of party-drug scene depicted, in hallucinatory fashion, by Gaspar Noé in his film Enter the Void. A caller claimed that Lucie had joined a cult and wished to sever all links with her past. Lucie's father, Tim, and her sister, Sophie, came to Tokyo to look for her and to raise awareness of her disappearance. Tim successfully lobbied Tony Blair, who persuaded the Japanese prime minister to make the case a priority. Offers of financial support were forthcoming but even some of his benefactors came to feel that Tim was enjoying not only the publicity he was generating but other aspects of the search, such as "checking out the girls" in the hostess bars where he was obliged to spend his time.

At various stages, the story assumes the form of eastern neon-noir. A private investigator tells Tim that he has made contact with Lucie's alleged abductors and can negotiate a ransom payment. Rumours from Tokyo's S&M underground lead to a torture chamber; without waiting for police back-up, a friend of Lucie's breaks in. Reading about the case, a woman who had worked as a hostess in Tokyo years earlier recalled how she woke up one day in an apartment by the sea with absolutely no memory of what had happened  but felt certain that she had been raped.

Eventually an immense web of mobile phone connections leads police to the home of Joji Obara who, they discover, had painstakingly documented and filmed his long history of knocking out women  Japanese and western  with chloroform and Rohypnol, and raping them. In at least one instance prior to Lucie's disappearance, this led to the victim's death (presumed by her family to be an accident).

As the focus shifts from Lucie's disappearance to the world of the suspect, readers may well start to regret having embarked on the book. Partly this is due to the grisly details of murder and disposal of a body; less palpably, in a way that will be familiar to anyone who saw the documentary Capturing the Friedmans, it has to do with the creep of moral corrosion and emotional collapse. Just as we are praying for a conviction and hoping that the family might at last find some solace, comes the revelation that Tim "had accepted half a million pounds from Obara, and signed a document questioning the evidence against him".

Japanese law puts such emphasis on a suspect's confession that a prosecution without one is comparatively rare (though once a prosecution is undertaken, conviction is a near certainty). Using his immense wealth (inherited, then augmented by buying and selling property during Japan's bubble economy of the 80s and early 90s) to obstruct and stall, Obara refuses to confess, even as the case against him seems overwhelming. "Demand me nothing, what you know, you know. From this time forth I never will speak word," says Iago at the close of Othello, denying the comfort of retrospective understanding. How to get beyond this dread end?

Not, Parry insists, by some spurious indictment of a society where crime rates and the risk of physical violence are far lower than in the UK. And yet in that society, a crime is "regarded not merely as the act of a criminal; in some deep sense, it originates within his family". To understand the family, Parry sets out a larger social and economic history, specifically of Korean immigrants such as Obara's parents. So the book is obliged to become a story "about Japan and Japanese people as a whole".

The family who earlier lost a daughter but believed Obara to be a good-natured citizen who happened to be with her at the time of her death remembered him looking at them  in Parry's words  "with an unreadable expression on his face". In a story defined by mutually reinforced racial stereotypes  complicated by the horrible irony that Lucie hated her "slanty eyes"  the author is only a synonym away from regarding him as inscrutable.

The "sad and mundane truth" Parry arrives at, ultimately, about Lucie's death "in a safe, yet complex, society" seems inescapable, yet he remains thwarted, even possibly (though this cannot be proved) harassed by Obara. Unable to arrive at an understanding of Obara's psychological make-up, Parry defines him by the effect he had on others: "a blizzard of darkness" that withers lives far beyond those with whom he came into contact. The taint spreads. Parry skilfully manipulates the narrative to keep the reader in a state of awful uncertainty about what will happen next.

By withholding the trial verdict and other details  including that "mundane" conclusion about Lucie's disappearance  am I using her death to avoid spoiling readers' potential what? Enjoyment? Enlightenment? Except there is no light, only darkness visible.

Geoff Dyer's Working The Room is published by Canongate

In classic tales of real-life murder  Truman Capote's In Cold Blood, John Berendt's Midnight in the Garden of Good and Evil, Gordon Burn's Somebody's Husband, Somebody's Son  the narrator isn't a detached observer but a man possessed. Where the crime reporter files his copy and gets a good night's sleep, the Capotes, Burns and Berendts can't let go till they've met everyone remotely involved with the case. Often they form relationships with the killers and/or the victims' families. And this makes their accounts disturbingly intimate, so that what seemed, at the outset, beyond all comprehension, becomes intelligible and even predictable, if no less grotesque.

For Richard Lloyd Parry, as Asia correspondent first for the Independent then the Times, writing articles on the long-running Lucie Blackman case was all part of his Tokyo day job. But Parry also found the case taking over his nights. He hung about in bars in the small hours, discovering a side to the city he'd never seen before. He made friends with Blackman's parents and siblings and received threats from lawyers acting for the man charged with her murder. He even had dreams of being the knight who rides to the dark tower to free the missing damsel. The result of his obsession is a compelling book, 10 years in the making, rich in intelligence and insight.

Tall and blonde, a 21-year-old former air stewardess, Lucie Blackman flew into Tokyo in May 2000 looking for fun, experience of a foreign culture, and a job that would help her pay off £6,000 of debt. Within days she was working as a bar hostess in Roppongi, the grungiest, least Japanese bit of Japan. Bright, friendly, all hair and nails, she was popular with some of the customers, though not as popular as her best friend Louise Phillips, whose idea it had been to come to Tokyo in the first place. In the club where they worked, being a hostess was more boring than sleazy: the job was to keep a man talking and drinking, not to service him sexually. But to earn bonuses and keep the boss happy, hostesses were also encouraged to go on dohan  dinner dates with besotted clients. Some of these clients were rich, including the one Blackman went off to have a late lunch with, by the seaside, 59 days after she arrived.

When she didn't return, Phillips knew at once that something was wrong: they'd been due to go out for the evening and Blackman would have phoned if there'd been a change of plan. But Phillips had no idea who the client was and the police showed little interest when she filed a missing person report: what with drugs, drink and the ups and downs of love affairs, it wasn't unusual for young foreigners in Tokyo to go awol for a few days. Besides, bar girls weren't respectable and as Parry says, "For many Japanese it was incomprehensible, indeed highly suspicious, that a woman should choose to give up a job as a stewardess at British Airways to become a bar hostess in Roppongi." A mysterious phone call  from a man who claimed that Blackman had joined a religious cult and wouldn't be returning  added to the confusion.

It was only when Blackman's father Tim and sister Sophie arrived in Tokyo and began holding daily press conferences that the police were stirred into action. Even then the investigation might not have taken off but for the G8 conference, which by chance was taking place in Japan that month.

Among the world leaders in attendance was Tony Blair, who met the Blackmans and raised the case with the Japanese prime minister. Suddenly the police came under pressure. But violent crime is comparatively rare in Japan and while omawari-san  the bobby on the beat  is a source of affection, the police force was ill-equipped to tackle serious offences. Even tracking the number of the mystery phone caller took months. The only clue to the man who had been with Blackman was that he dressed smartly, spoke good English and had driven her to the seaside. This rang bells with several women, who recalled a man  Yoji, or Kuji, their names for him varied  who'd taken them to his seaside apartment, then used chloroform or drugs to knock them out.

Most had groggily departed the following morning, aware that they must have been raped but disinclined to report him. After Blackman's disappearance they belatedly came forward, and though the police were slow to piece together their stories they eventually, three months later, made an arrest.

The man in custody was Joji Obara, an affluent middle-aged businessman and, like Blackman, an outsider  a Zainichi or ethnic Korean. He used aliases, hated photographs, had no friends, and despite owning several properties moved through life without a trace, a master of anonymity. But in the flat police found videotapes he'd made of his rapes. Among the victims caught on film was an Australian, Carita Ridgway, who, unlike the others, had not woken up: her death from liver failure had seemed inexplicable nine years earlier but Obara could now be charged with her rape and murder. Though there was no film of Blackman, the fact he'd purchased a hand saw, chainsaw and plastic bags in the days after her disappearance made him the obvious suspect. But police incompetence in searching the area meant that it was seven months before her dismembered corpse was found in a small cave on the beach, 250 yards from his flat.

Behind the scenes, or occasionally in public, Blackman's family reacted to their loss in different ways, and Parry, who came to know them well (her father especially), describes the fallout in some detail. Blackman's mother Jane took refuge in mediums and psychics, and blamed her ex-husband for the tragedy. He buried his grief by courting the press and setting up a charitable trust in his daughter's name.

Sophie attempted suicide the night after the family had buried Lucie's ashes and spent nine months in a psychiatric hospital. Rupert, Lucie's younger brother, dropped out of university and had a nervous breakdown. The toll was heavy  and the trial did nothing to relieve it.

Despite the absence of DNA evidence, the case should have been straightforward. But the Japanese legal system relies heavily on a confession from the accused, and Obara stubbornly denied Blackman's murder. In court, over the next six years (in Japan, trial hearings take place only once a month), the judge listened to overwhelming circumstantial evidence against him. And though Obara's lawyers tried desperately to shift the blame, their case was so preposterous that no one, least of all Parry, gave them a chance. Yet astonishingly, while given a life sentence for murdering Ridgway and for eight other rapes, Obara was acquitted of Blackman's murder.

Little wonder that the Blackman family then unravelled. It didn't help that, towards the end of the trial, to the disgust of his ex-wife and the British media, Blackman's father accepted a half-million pound "consolation" payment from Obara ("consolation" rather than atonement, since Obara would not own up to any guilt). For Parry, too, the aftermath of the trial was bizarre. First Obara tried to sue him for libel. Then a large envelope was delivered to his home, containing photographs of him taken by a stalker and accusing him of insulting the Japanese royal family. Finally, in December 2008, after various appeals, Obara was found guilty of abducting, drugging, dismembering and disposing of Blackman, if not  since no post-mortem could identify the cause of death  of killing her. But he is still fighting legal battles to this day, and the story won't ever be over for the Blackmans.

In brutal thrillers, Northrop Frye said, "We come as close as it is normally possible for art to come to the pure self-righteousness of the lynching mob." But what's heartening about Parry's book is its refusal of hysteria or demonisation. This isn't just the tale of a murder case but a book that sheds light on Japan, on families, on the media, and (in ways that bring back memories of the Yorkshire Ripper case) on the insidious effects of misogyny. Open-minded and sympathetic despite being driven half mad, Parry is the best kind of narrator. It may be that the story won't ever let him go, but he tells it with such clarity and compassion that catharsis is the least he deserves.

Blake Morrison's The Last Weekend is published by Chatto & Windus.