When I was at university in the late 80s, the influence of The Madwoman in the Attic, Sandra Gilbert and Susan Gubar's milestone feminist study of the Victorian imagination, could be felt in every corner of the department of English: liberating breeze or poisonous miasma, depending on your point of view. As a result, our studies were onerous. It wasn't enough to read Jane Eyre, Villette and Middlemarch. Beside them on your desk would be a teetering stack of books about Victorian attitudes to sex and science, to marriage and mesmerism, to geology and phrenology and the female malady  plus a pile of minor novels, too. The sensational and monstrously swollen stories of Mrs Henry Wood and Mary Braddon were, we were told, ripe for reconsideration. At the end of a week of this, exhaustion and terror combining to bring me halfway to a swoon, I felt like the heroine of one of these febrile tales myself: dazed, confused and in desperate need of, if not smelling salts, then at least a Twix.

Future students are going to have it easier. They will need only to turn to Mrs Robinson's Disgrace. Yes, on the surface of it, Kate Summerscale's new book is a straightforward account of misplaced love and misguided betrayal. Like her award-winning The Suspicions of Mr Whicher, it blows the dust off long-forgotten people and events, and sees in them the seeds of literary inspiration. If Mr Whicher was a model for writers of detective stories from Wilkie Collins on, Mrs Robinson is a real-life Lady Isabel Carlyle, the sexually obsessed wife in Mrs Henry Wood's 1861 bestseller, East Lynne. (She also resembles, as her publisher trumpets, Emma Bovary, though she surely wasn't that character's inspiration: the dates don't tally.) But Mrs Robinson's Disgrace is also a vast section of Victorian thought in microcosm, a breathtaking achievement its author pulls off almost casually in (discounting her extensive notes and bibliography) 226 scant pages.

You would not believe the walk-on parts her narrative turns up. Dickens strolls by, and Darwin, while the novelist Catherine Crowe is at one point found wandering the streets of Edinburgh, mad and stark-naked. Summerscale's heroine, Isabella Robinson, is an acquaintance of the publisher Robert Chambers, whose bestselling Vestiges of the Natural History of Creation was a daring, proto-evolutionary account of the Earth's formation  and she was a busy correspondent of George Combe, the phrenologist whose fame was such that Queen Victoria invited him to examine the head of her nine-year-old son, the Prince of Wales.

Isabella's putative lover, Edward Lane, is the proprietor of a fashionable spa, Moor Park, at which patients tried the new fad for hydropathy, and his brother-in-law, George Drysdale, is the author of Physical, Sexual and Natural Religion, a guide to contraception and sexual disease, which argued that desire was natural and masturbation no substitute for the real thing, since it kept the natural passions "pent up in the gloomy caverns of the mind". Against this enlightened backdrop, her infatuation with Lane, the cause of her subsequent "disgrace", seems almost mundane  until, that is, you remember that beyond her circle the world was still corseted in fear and loathing. In 1850 a woman who desired to have intercourse with a man other than her husband was usually classified as suffering from sexual mania, a form of madness. For Isabella this must have been bewildering: the pull of free thought and the push of repression. It would, I think, have crushed a lesser woman altogether.

The story begins in 1844, when Isabella, a widow from a good family, married Henry Robinson, a civil engineer, on his third proposal. "I suffered my scruples & dislike to be talked away by others," she later explained, "& with my eyes almost open I walked into the bonds of a dreaded wedlock like one fated." The marriage was advantageous to them both: as the mother of a small child, Isabella could not afford to be picky, while for Henry it brought status and money. For the sake of a quiet life she signed over to him a settlement, made on her by her father, of £5,000.

Soon after, the couple moved to Edinburgh, and it was there that Isabella met Edward Lane, a "fascinating" young man who was training to be a doctor. Lane was 10 years her junior and in possession of a devoted wife, Mary, but he was a charming and able conversationalist, and he and Isabella, an intelligent and curious woman for all that she seems also to have suffered from chronic ennui, sometimes talked together about new ideas. In particular they discussed the loss of their religious faith. "I said that the grandeur of truth made up to me for relinquished hopes," she wrote in her diary afterwards.

Isabella quickly became infatuated with Lane, a passion she detailed feverishly in the pages of her diary. When Edward was responsive  or at least kind enough occasionally to answer her letters  she was on top of the world. When he was cold  when, or so the reader senses, he felt mildly alarmed by her attentions  she would lapse into headaches, listlessness and depression. The pity of it is that it wasn't until four years later (and after she had enjoyed similarly unreciprocated, if less ardent, infatuations with two of her sons' tutors) that anything of substance happened between them. In 1854 at Moor Park, Surrey, where Lane now offered patients the "water cure" (Darwin was treated for his dyspepsia with a douche aimed at his abdomen), the couple finally kissed. Isabella was in ecstasy. Was the relationship consummated? There is no evidence that it was. In her diaries she talks of her "half-realised" bliss in Edward's arms; her tone is one of sexual frustration rather than satisfaction.

But this is, and was, a moot point. In 1855, after her relationship with Lane had again cooled, Isabella was taken ill, probably with diphtheria. Looking into her room one day, Henry heard his delirious wife mutter the names of several men. He went to her desk, opened her diary and learned both of her loathing for him (not that he was exactly a devoted husband; he had a mistress and two illegitimate daughters) and of her infatuation with Lane. As soon as she was well again he told her he intended to sue for divorce.

Summerscale devotes the second half of her book to this decision and its consequences for all involved. I won't reveal who won, or how, but it is riveting to watch the new Court of Divorce and Matrimonial Cases and the newspapers which followed the case so avidly wrestling with the only serious piece of evidence available to them: Isabella's purloined diary. Was it a true account of events? Or was it the work of a fantasist? And if it was true, wasn't it a clear sign of sexual mania? Surely only a madwoman would put such shameful thoughts in writing? Summerscale's account of this court case is faultless; her seemingly inexhaustible capacity for research renders what could be tedious and dry vividly alive. She will tell you how a judge liked to wave his lorgnette, as well as what he said in court. But she deals in fact, not supposition, so when the narrative lags, or when gaps inevitably appear, she uses reading rather than imagination to enliven proceedings: the miniature essays she drops into the text about diary writing and its place in Victorian culture; about hydropathy, alienism and so many other things besides, are lively, thoughtful, and remind you how truly peculiar the Victorians could be.

There were times, I'll admit, when I longed for her to take sides, to rail against the supposedly liberal men who, on discovering that they too appeared in Mrs Robinson's diary, worried for their own reputations without ever losing a moment's sleep over hers. She is sometimes, for my taste, a little too careful, too precise, too cool. And I would have liked an afterword, telling us how she stumbled on this curious story in the first place. But mostly I'm all admiration: she has turned a sepia photograph, curling and tattered, into a film that runs through the mind in glorious and unimpeachable Technicolor.

The chief exhibits from Kate Summerscale's deservedly bestselling and prizewinning last book, The Suspicions of Mr Whicher, are still hanging about rather grotesquely in my mind. The mutilated body of a boy in the water closet, a sash window slightly open, a bloodstained nightdress stuffed into the boiler. Summerscale's reconstruction of the infamous murder investigation at Road Hill House was also a cross-examination of Victorian domestic life and its most disturbing secrets. Now, in Mrs Robinson's Disgrace, she uses the same techniques of historical sleuthing to reconstruct the events that brought one couple to the divorce courts in 1858. As before, she follows the clues outwards from this one case to the larger anxieties, prejudices and cover-ups that shaped it. Summerscale puts Victorian middle-class society back in the dock, and again it is both horrifying and salutary to follow the questioning from the gallery.

The chief exhibit this time is a diary. No blood, no corpse in the privy, just an ordinary Letts diary of the sort that is currently half price in Smith's. But this too is a relic of passions that could not be contained and which are exposed in the end to the scrutiny of a nation. The unfortunate diarist, Isabella Robinson, fell in love with a man who was not her husband and wrote her feelings down. That was her crime, that was her ruin, and that was all it took to cause a scandal of major proportions. Her husband, the industrialist Henry Robinson, had a range of mistresses and two illegitimate children, but no matter. For a woman the standards were different. After all, she didn't even own her diary.

The man she loved was the married doctor Edward Lane, pioneer homeopath and proprietor of an advanced hydropathy establishment in Surrey where patients were prescribed a bewildering number of different kinds of bath. Isabella spent time at Edward's spa, and in his study, but did they embark on an affair? Nothing in Edward's letters proved it  he was too careful a correspondent. Isabella, on the other hand, wrote in her diary a day-by-day narrative of her erotic longing and the dreamed-of reciprocation that began one afternoon in the Surrey countryside when Edward turned to her on the plaid picnic rug and kissed her.

In her diary Isabella was the heroine of her own romantic novel. She described the misery of her marriage to the insensitive Henry, her ennui and entrapment, the great redeeming joy of evenings spent reading poetry with Edward. She wrote out her fantasies and expressed the full force of her desire. She stopped just short of recording everything ("I rested among the dry fern. I shall not state what followed"), but she included in her diary much more than was licensed in any contemporary fiction. In France, Gustave Flaubert was at work on his great novel of adultery, Madame Bovary, but it would be banned from publication in England, deemed too repulsive and corrupting for English eyes.

Henry Robinson was one of the first to sue for divorce under the new Matrimonial Causes Act of 1857, a landmark act that for the first time made divorce a possibility for middle-class couples. If Mr Whicher's investigation at Road Hill was the original whodunnit, Robinson v Robinson & Lane was the "original" divorce, the ancestor of more than 100,000 divorces that took place in the UK in 2010 alone.

And what an agonising mess it was. Isabella's diary was proof of her lustfulness but it did not prove anything conclusively against Edward. Could it be used against her and not against him? The only way to save Edward's reputation (as everyone was eager to do, at Isabella's expense) was to discredit the diary as the raving of a sex-obsessed lunatic. By a skewed logic more perverse than anything Isabella could dream up, a sane woman was now reinvented as an erotomaniac driven mad by a conveniently identified uterine disease.

At every stage this "real-life" story is a skein of fictions. Coleridge and Shelley taught Isabella what a love affair might be, and how she might construct herself as its heroine. The lawyers, too, learned their lines from literature. In court they fantasised a gothic tale in which Isabella's madness poisoned a whole respectable milieu. Summerscale is a subtle interpreter of the interplay between action and literary imagination, as was clear in Mr Whicher. A large part of her fascination with the Road Hill case lay in its influence on subsequent detective fiction. If the literary connections in Mrs Robinson are less compelling (and it should be said that Isabella is not going to vie with Emma Bovary for literary immortality), they are crucial to the vivid anatomy of an 1850s mind.

Summerscale might have been more upfront about her own methods of narration in a book so concerned with reading, writing and the interpretation of documents. There are potentially significant gaps and doubts in a story she strives hard to render as a polished whole. It comes as a surprise, for example, late in the day that Summerscale has not read Isabella's diary, the fateful book having been lost or, more likely, destroyed. She has had to piece together her account from extracts published in legal reports. She has made careful use of correspondence, but how representative are the letters that survive? Foraging in the footnotes in an effort to find out, I couldn't help feeling that discussion of these matters would have left us better equipped to read Isabella and her times.

As a guide to mid-Victorian cultural life, however, Summerscale is simply superb, and she sets a fine example of what cultural history can do. Isabella has her head examined by a phrenologist, so we get a miniature history of phrenology and its implications. (A large cerebellum meant excess "amativeness"; Isabella's cerebellum was very large indeed.) To understand Edward and his Moor Park spa we need to know about hydropathy, so we go on a course in alternative medicine and curative bathing (chair baths, hot air baths, wet towel baths, secretly sensuous baths).

In other hands, admittedly, this might become tiresome. But Summerscale has a knack of judging just how much we want to know. So she keeps adding strands to the web: divorce law, diary-writing, Victorian dream theory, gentlemen's advice on the advantages of erotic encounters in bumpy carriages. She knows that the settings, too, are eloquent: Henry's big white villa in Caversham where nobody is happy, the sandy soil of the Surrey hills, the precise qualities of Edward's study with its many doors, and the foul smells from the Thames that filter into a hot Westminster courtroom at the stinking centre of the British empire.

Sensing a silence or slight misunderstanding between two characters, Summerscale prods a bit, and the door flies open to a whole new set of stories. It's like watching someone going straight to the secret compartment in a many-drawered cabinet. Edward's friend and brother-in-law, George Drysdale, needn't have figured at all in Isabella's story, except that he turns out to cast a shadow across the whole affair. He faked his own death out of shame for his sexual fixations, resurrected himself, failed to cauterise his penis, and went on to write the first guide to contraception. That's the kind of obscure but astonishing life story we keep glimpsing in the background.

And we glimpse too, at a distance, famous people going about their business, like Hamlet in Rosencrantz and Guildenstern Are Dead. When not decoding Isabella, the phrenologist is feeling George Eliot's cerebellum; Charles Dickens is leaving Boulogne just as Isabella gets there; and who should be strolling at Moor Park but Charles Darwin, so relaxed by his bath treatments that he said he "did not care one penny how any of the birds and beasts had been formed". He didn't stop caring for long, of course: he was at work that year on The Origin of Species, refining his theory of evolution even as Isabella reconciled herself to atheism and wondered what the absence of God might mean for the future of sexual relations.

At every turn Isabella's experience is contiguous with that of the people who were deciphering and shaping her world. But ultimately it is Isabella herself who stands as exhibit A in this engrossing investigation of a society casting judgment on itself and trying, with much confusion, to make up the rules.

 Alexandra Harris's Virginia Woolf is published by Thames & Hudson.