It was one of the most notorious murders of its age. Galvanizing early 20th-century Britain and, before long, the world, it involved a patrician victim, stolen diamonds, a transatlantic manhunt, and a cunning maidservant who knew far more than she could ever be persuaded to tell. It was, Sir Arthur Conan Doyle wrote in 1912, “as brutal and callous a crime as has ever been recorded in those black annals in which the criminologist finds the materials for his study.”

But for all its dark drama, and for all the thousands of words Conan Doyle would write about it, his account of this murder was no Sherlock Holmes tale. It concerned an actual case: a killing for which an innocent man was pursued, tried, convicted, and nearly hanged. This miscarriage of justice would, in his words, “remain immortal in the classics of crime as the supreme example of official incompetence and obstinacy.” It would also consume him — as private investigator, public crusader, and ardent nonfiction chronicler — for the last two decades of his life.

The case, which has been called the Scottish Dreyfus affair, centered on the murder of a wealthy Glasgow woman. Just before Christmas 1908, Marion Gilchrist, a furtive 82-year-old who kept a vast collection of jewels secreted in her flat, was found brutally bludgeoned to death. The police declared that robbery was the motive, for Miss Gilchrist’s maid told them that a prized brooch — a gold crescent moon set along its length with diamonds — was missing.

Oscar Slater, 1908. Photo by Hulton Archive/Getty.

Glasgow police detectives soon settled on a suspect: 36-year-old Oscar Slater, a German Jewish cardsharp recently arrived in the city who was reported to have pawned a diamond crescent brooch. Under pressure to close the case — and happy to rid Glasgow of an immigrant Jew of dubious livelihood — they pursued him nearly into the grave. With Miss Gilchrist’s maid and two other witnesses in tow, they followed Slater by ship to New York, where he had sailed on a long-planned voyage, and extradited him to Scotland. His very name would become so notorious there that for years afterward the phrase “See you Oscar” was Glasgow rhyming slang for “See you later” — as in “See you later, Oscar Slater.”

In May 1909, after a jury deliberated for barely an hour, Slater was found guilty and sentenced to death. The pro­nouncement had a terrible finality: There was no criminal ap­peals court in Scotland then. (Pardons, when they were occasionally granted, were by prerogative of the British mon­arch.) There was enough public unease about the verdict, however, that a petition for clemency garnered 20,000 signatures. Forty-eight hours before Slater was to mount the scaffold — he had already made arrangements for his own burial — King Edward VII commuted his sentence to life at hard labor.

For the next 18 and a half years, Slater remained imprisoned, largely forgotten, on a barren, windswept outcropping in the north of the country, in a place that would one day be known as “Scotland’s gulag”: His Majesty’s Prison Peterhead, a stark Victorian fortress erected in 1888. Day after day, in bone-rattling cold and blistering heat, Slater hewed immense blocks of granite; endured a Dickensian diet of bread, broth, and gruel; and often languished in solitary confinement. Had he passed the 20-year mark behind bars, he said, he would have taken his own life.

Then, in 1927, Slater was abruptly released; his conviction was quashed the next year. What set these events in motion was a secret message he had managed to smuggle out of prison in 1925. That message — an impassioned plea for help — was directed at the one man he believed could save him: Arthur Conan Doyle.

Sir Arthur Conan Doyle, 1925. Photo: General Photographic Agency/Getty

Writer, physician, worldwide luminary, champion of the downtrodden, Conan Doyle had believed in Slater’s innocence almost from the start. First joining the case publicly in 1912, he turned his formidable powers to the effort to free him, dissecting the conduct of police and prosecution with Holmesian acumen.

That the story does not end with Slater’s death in prison owes chiefly to Conan Doyle. As investigator, author, publisher, and backroom broker in the loftiest corridors of British power, he is credited with having done more than anyone else to win Slater’s freedom in a case that many observers deemed hopeless. “The Slater affair,” one of Conan Doyle’s biographers has written, “was to give Conan Doyle the chance to play a similar part in England to Zola’s intervention in the Dreyfus affair in France” — invoking the great writer’s support for Alfred Dreyfus, a Jewish officer in the French army falsely convicted of treason in 1894 amid a climate of roiling anti-Semitism.

Conan Doyle remains venerated today as a crime writer, but he is less well remembered as a crusader — “that paladin of lost causes,” as one British criminologist memorably described him. By the time he died, in 1930, at 71, he had twice run for Parliament (without success) and had championed a string of causes, including divorce reform; the exposure of Belgian atrocities in the Congo; clemency for his friend Roger Casement, convicted of treason; and, in his later years, incongruous as it might seem for a man of such exquisite reason, the existence of the afterlife and the spirit world. Renowned as the creator of Sherlock Holmes, very likely the most famous character in Western letters, Conan Doyle was repeatedly beseeched by members of the public to solve real-life mysteries — deaths, disappearances, and the like — performing successful feats of amateur detection on more than one occasion.

By the time he cast his lot with Slater, Conan Doyle had helped right another wrongful conviction born of bigotry, that of George Edalji, an Anglo-Indian lawyer imprisoned for maiming livestock. Conan Doyle’s personal investigation of that case is the subject of a spate of nonfiction books and inspired Julian Barnes’ acclaimed 2005 novel, Arthur and George.

But the Slater story, though it involves homicide, remains less well known, perhaps because the case is more complex than any other Conan Doyle tackled. For one thing, it lacks the stainless suspect and moral absolutes that the Edalji case presented. Where George Edalji was an educated professional man of unimpeachable character, Oscar Slater was an affable Continental rascal: a habitué of music halls and gambling rooms and, it was alleged (though never proved), a pimp. Conan Doyle himself thought Slater a blackguard: “a disreputable, rolling-stone of a man,” he called him — seven words that speak volumes about the reflexive cultural assumptions of his era.

In addition, Conan Doyle, creator of the ultra-rationalist Holmes, had become something of a laughingstock in the last decades of his life for his vigorous endorsement of spiritualism. As a result, the press and public were inclined to regard any cause to which he attached himself, Slater’s included, with skepticism, if not out­right derision.

Yet the case was Conan Doyle’s last stand as a true crime investigator, and a remarkable stand it was. The story of his long effort to free Slater throws into relief the singular temperament that let Conan Doyle light the age in which he lived: a readi­ness to wade into battle, a sense of honor so intense that it trumped personal antipathies, and a talent for rational investi­gation that far outstripped that of the police. Where today many wrongful convictions have been overturned through DNA analysis, Conan Doyle managed to free Slater with little more than minute observation and rigorous logic — precisely the kind of brainwork that had made his hero world famous.