Recently, it seems to have become the mark of a really A-list historical disaster that it has featured on Doctor Who. So it was, for instance, during the most recent season that the Doctor, fresh from having prevented a Titanic-shaped spaceship from crash-landing on London, arrived in the Roman town of Pompeii. The date, of course, was AD79: the fateful year in which Mount Vesuvius spectacularly blew.

The episode was to prove a witty and knowing manipulation of the gap between many of the favourite myths about life in Roman times, and what the evidence for them from Pompeii actually suggests. Particularly cherishable was its gold medal-winning effort in what has become celebrated among classicists as the "dormouse test": the principle that the longer it takes for the delicacy to be mentioned in a drama set in ancient Rome, the more authentic the reconstruction is likely to be. In Doctor Who, it took a peckish Pompeiian a bare two minutes to demand said dormouse.

Now, in a new book, the inventor of that test, the Cambridge classicist Mary Beard, has provided her own reconstruction of Pompeii. As her concern with dormice suggests, she is a historian who has always been fascinated by the stereotypes we have of ancient Rome: both how they came into being, and how valid they are likely to be. Indeed, such is the relish with which she goes about her myth-busting that it seems to reflect not scorn, but rather a wry affection for the myths themselves. What better theme for her, then, than Pompeii? After all, ever since excavations of the buried city began in the mid-18th century, it has provided us with the nearest thing we are ever likely to have to a freeze-frame from the ancient past - and yet many of our presumptions about what it can teach us turn out, on closer inspection, to crumble to dust. Hence what Beard, coining another catchy formula, terms the "Pompeii paradox": "that we simultaneously know a huge amount and very little about ancient life there".

Part of the problem is that the city's population was not, as Doctor Who suggested, wiped out in a single day amid a wholly unanticipated cataclysm. On the contrary: all but the brave or foolhardy had already fled their homes before the climactic pyroclastic surge descended from Vesuvius to entomb the remaining Pompeiians for good. The implications of this for archaeologists and historians, as Beard makes clear in a typically invigorating chapter, are profound: for what we have frozen in Pompeii is not a scene from everyday life, but rather a place that was already well on its way to becoming a ghost town. The denuded character of the houses bears witness less to a taste for minimalism, than to wholesale evacuation.

Nor is that the only complicating factor. The very fame of Pompeii, the fact that it has been the most famous archaeological site in the world for so long, means that centuries' worth of repairs and restorations have added their own overlay to the Roman originals. Emblematic of this process is the so-called Villa of the Mysteries, which seemingly portrays a Dionysiac initiation rite on its walls: it certainly sparkles, and yet, as Beard points out, "that sparkle is not an ancient one", but rather the consequence of a touching-up in 1909. Even more insidious is the sheer impact of the villa's name: for the truth is that we cannot be certain that its friezes illustrate a mystery ritual at all. It is not only the dust and ash of Vesuvius, then, that historians need to excavate in their attempts to see through to the classical past of Pompeii, but also, in many cases, the theories and presumptions of the excavators who went before them.

Yet if Beard's book is a vivid demonstration that sceptical scholarship can provide as gripping a read as sensationalism, that does not mean that the author shrinks from providing her own exploration of what life might actually have been like. In many ways, it is an extraordinarily vivid one. Her Pompeii is a city in which dogs howl, late-night drunks carouse, and everyone has bad breath. It is a city in which, as Beard points out with some glee, a household of perhaps some 30 people had only a single lavatory between them, and the crowds at the amphitheatre not even that: "20,000 people and nowhere but the stairs and corridors to take a piss." Above all, it is a city that is infinitely messier and less systematised than the guide books ever allow: where the presence of sexually explicit graffiti on a wall does not necessarily suggest a brothel, and where the baths, as well as providing a bather with "a place of wonder, pleasure and beauty", were so polluted that "they might also have killed him".

And how does Beard herself fare with the dormouse test? Sure enough, some three-quarters of the way through Pompeii, we are shown the illustration of a "dormouse-jar": a curious pot in which the wretched rodents could be kept alive while being fattened up. It is an image that perfectly sums up the portrait of Pompeii we are given in this learned and fascinating book: a myth that is not wholly a myth, but something even more remarkable and strange.

· Tom Holland's Millennium is published by Little, Brown.