In Conan Doyle for the Defense, Margalit Fox brings to life a forgotten cause célèbre in this page-turning account of how mystery-writer-turned-real life sleuth Arthur Conan Doyle helped exonerate a man who was wrongfully convicted of murder. Fox takes us inside the famous crime and how Conan Doyle played a part in the outcome.

It was one of the most sensational crimes—and most scandalous wrongful convictions—of the 20th century, a case that would be known as the Scottish Dreyfus affair. It involved a savage murder, stolen jewels, an international manhunt and a wily maidservant who went to her grave knowing far more about the killing than she would ever disclose.

Even more remarkably, it involved the world’s foremost writer of detective fiction, playing real-life detective on a case in which the stakes could scarcely be higher—a case, he wrote, that was a “disgraceful frame-up, in which stupidity and dishonesty played and equal part.”

Just before Christmas 1908, Marion Gilchrist, a wealthy 82-year-old Scotswoman, was violently murdered in her Glasgow home. Robbery appeared to have been the motive, although Miss Gilchrist’s maid, Helen Lambie, told the police that only a single item was missing: a valuable gold brooch, shaped like a crescent moon and set along its length with diamonds.

The Glasgow police soon settled on a suspect: 36-year-old Oscar Slater, an immigrant German Jewish gambler who had recently pawned a similar brooch. Though the police learned within a week that Slater was innocent—Miss Gilchrist’s brooch had a single row of diamonds, whereas Slater’s had three—they pursued him all the same: They were under immense pressure to solve the case, and Slater was precisely the type of man (foreign, Jewish, of dubious livelihood) they wanted off Glasgow’s streets anyway.

Police and prosecutors unashamedly built a case against Slater through witness tampering, the subornation of perjury and the suppression of exculpatory evidence. Sailing with Lambie and two other witnesses to New York, where Slater had gone on a long-planned trip (a voyage, they argued, that was a clear indication of flight), they extradited him to Scotland, where he stood trial in the spring of 1909. After a jury deliberated for barely an hour, he was convicted and sentenced to hang.

There was sufficient public unease about the verdict, however, that a petition to commute Slater’s death sentence garnered 20,000 signatures. Forty-eight hours before he was to be led to the gallows (he had, chillingly, already made arrangements for his own burial), his sentence was commuted to life at hard labor on order of King Edward VII. Remanded to His Majesty’s Prison Peterhead, a Victorian fortress in the north of the country that would one day be known as “Scotland’s gulag,” he remained, largely forgotten, for almost two decades. Had he passed the twenty-year mark behind bars, Slater said, he would have taken his own life. Then, in 1925, he managed to smuggle out a message to the one man who could save him: Sir Arthur Conan Doyle.

He could not have enlisted a better advocate. Though Conan Doyle is renowned today as the author of the Sherlock Holmes stories, he was then equally well known as an indefatigable champion of the underdog. One of the most famous men in Britain, if not the world, Conan Doyle brought a string of injustices—among them divorce reform, Belgian atrocities in the Congo and clemency for his friend Roger Casement, convicted of treason—to wide attention through his books, articles, lectures and ceaseless tide of letters to newspapers. He even ran twice for Parliament, though he lost both times.

Above all, he possessed the acute observational skills and impeccably logical cast of mind that would let him dismantle the case against Slater plank by plank—attributes that stemmed not only from his work as a crime writer but also from his training as a medical doctor. As I was fascinated to learn while researching Conan Doyle for the Defense, the art of crime detection and the art of medical diagnosis are strikingly similar. Both often start with a body. Both seek an elusive quarry: a criminal, a germ, a virus. Both use the minute analysis of barely discernible clues to reconstruct a sequence of past events. Both seek to restore a state of safety and order.

Outraged by Slater’s fate, Conan Doyle had been advocating publicly on his behalf since 1912. But he soon found, he wrote, that “I was up against a ring of political lawyers who could not give away the police without also giving away themselves.” And so, despite his formidable intellect, influence, and energy, the Slater case endured as one of the most attenuated judicial tragedies of its day. Now, in 1925, Slater’s smuggled message—furled into a tiny pellet and secreted under the dentures of a paroled fellow convict—moved Conan Doyle to take up his cause one last time.

His task, as he knew from the start, was not to find out whodunit but to prove who had not. (Although Conan Doyle was too diplomatic to say so publicly, his private writings reveal that he formed a strong opinion as to the real killer’s identity.) By combing through hundreds of pages Conan Doyle’s published writings unpublished correspondence, which I had the pleasure of seeing in archives throughout Britain, I was able to recreate his modus operandi in the Slater affair.

Conan Doyle brought to the case an investigative process that is can truly be called Holmesian: isolating the authentically meaningful clues from the wash of evidentiary noise; homing in on telling negative evidence (why did Lambie behave so strangely on finding her mistress murdered?); and illuminating the slipshod reasoning, overt prejudices and outright fabrications of police and prosecutors. In the end, after years of investigation, agitation and publication, he won Slater’s freedom. Slater was released from Peterhead in 1927; his conviction was quashed the next year.

My archival research also helped animate Slater himself, who in previous accounts of the case was a haunting absence, a cipher at the center of his own story. But his correspondence archived at the National Records of Scotland in Edinburgh—a bulging file containing every letter he sent and received in eighteen and a half years in prison—reveals a charming, roguish, bewildered immigrant Everyman attempting to resign himself to his fate without giving way to despair entirely.

In a 1912 letter to his parents in Germany, Slater wrote, “My only hope which I still have is, that the murderer, before closing his eyes forever and forced by remorses[sic], will make a confession before witnesses.” That wish would never come to pass. Though Miss Gilchrist’s maid, Helen Lambie, told police that she had seen another man—a highly placed member of Glasgow society—leave the crime scene, she later recanted that statement under duress from the prosecution, and the case against Slater proceeded apace. The murder remains unsolved to this day.

In the end, Conan Doyle’s triumph in the Slater case reveals a great deal that was commendable about the age: valor, fair play and wielding of the scientific method against a system that was willing to hound an innocent man nearly into the grave. What I had not anticipated when I began work on Conan Doyle for the Defense six years ago, however, was that the case—with its pervasive undercurrent of nationalism, anti-Semitism and xenophobia—would also be a dark, uncannily accurate mirror of the way we live now.