However prepared you are, the first time you descend into the Tokyo subway during the morning rush hour, it's still a shock. There's nothing frantic, disorderly or impolite when it comes to boarding a train. But the number of people stoically squeezing into each carriage is extraordinary: it's not a question of finding somewhere to stand where you won't step on your neighbours, you are simply thrust up against a flexible wall of silent, smartly dressed, odourless humanity.
Claustrophobic, certainly, but in the event it's an oddly comforting sensation. No one complains or sticks out an awkward elbow; everyone is accommodated, the doors slide shut, the train moves off. Before long, eyes closed, you, too, are drifting off to sleep, securely wedged upright within this rocking cradle - at least until the next stop. It is a very Japanese moment.
The notion that deliberate harm could befall anyone cocooned on such a train is profoundly disturbing. But on the morning of Monday 20 March 1995, five senior members of the Aum Shinrikyo cult board different trains, drop plastic bags to the floor and puncture them with the sharpened tips of their umbrellas. They leave. People in the carriage start to cough. Leaking out of the bags is sarin - the nerve gas that was invented by Nazi scientists and later used by Iraq against Iran and the Kurds. A drop of sarin the size of a pinhead is sufficient to kill a person: within a matter of hours, 12 Tokyo commuters and subway staff will be dead, and another 5,000 will be less seriously affected.
Haruki Murakami wanted to give the victims of the attack their say, and has assembled Underground out of a series of interviews conducted during 1996. Despite the apparently huge nature of such a task, only 60 of the thousands of victims were willing to respond. Some railway staff didn't want to remember the day when everything went wrong. Other people were scared of attracting the attention of Aum or the disapproval of their employers. But Murakami persevered, and talked to rail attendants, hospital doctors, bereaved parents and to people who climbed out of the subway apparently unscathed only to find the sky going dark as their pupils contracted. He interviewed a victim left severely paralysed - in a wheelchair, with no memory, tended to by her brother - and, in the second half of the book, ex-members of the Aum cult.
These testimonies, deftly assembled, are individually powerful; together, they resonate in unexpected ways, as the same events unfold from different perspectives. What did sarin smell like? What was its immediate effect? Well, "more of a sensation, not a smell, a 'suffocatingness'"; "it was as if the air was running out"; "it was as if the air itself had shut down, even time had shut down". Piecing together these overlapping memories, we pick out people whose fate we can anticipate: the poignant images of the station attendants who, as they clean up the sarin-drenched newspapers, are already dying; or the helpful woman who lends a red handkerchief to a driver so that he can wave it as he speeds through traffic to the hospital.
For Murakami, returning to Japan after a long self-imposed exile in America, the Aum cult "shows us a distorted image of ourselves in a manner none of us could have foreseen". And Murakami candidly confesses that he seized on the gas attack as an opportunity for writing: it gave him a chance to understand and reconnect with Japan "at a deeper level".
His critique of Japan's stifling institutions, the endless machinations of office - "inner-circle-upon-inner-circle" - is as persuasive here as it is in his increasingly political fiction. But we are perhaps less persuaded by his analysis than by his openness - by the way he finds cult members sympathetic misfits, solemnly if misguidedly working out important questions. The terrible flaw he identifies in them is blind faith in another person, in a guru. Even if despicable, the leader of Aum, Shoko Asahara, can be viewed as "a master storyteller who proved capable of anticipating the mood of his times".
Murakami is a different kind of storyteller, who knows there isn't only one narrative to tell. He doesn't practise any form of editorial "triage"; unlike the emergency doctors, he has no need to prioritise, to pass over the less serious cases. Instead, he finds ways to accommodate them all. His attractively modest approach - "words can be practically useless at times, but as a writer they're all I have" - turns this account of a nightmare into a work of consolation.
Julian Loose is an editorial director of Faber & Faber