“Never in my wildest dreams did I think there’d be an American publisher for a book about an obscure British politician,” John Preston says, laughing, as he busies himself at the coffee maker in his sunny West London kitchen. “But in a funny sort of way, it now strikes me, it’s such an English story that it becomes an advantage. The British establishment and Parliament—purportedly the upper echelons of society; cloak and dagger; attempted murder: There’s a good story there.” I make a comparison to a British TV detective drama, and he agrees: “In a funny way, it does tap in to Midsomer Murders!”

The story in question is A Very English Scandal: Sex, Lies and a Murder Plot in the Houses of Parliament (Other Press, Oct.), and Preston, a seasoned journalist and novelist, was a young stagehand when the events the book is based on started making headlines. Born and brought up in southern England, he hated boarding school and dropped out of university to join the theater, eventually “drifting into journalism, it being of the few professions that accepted people with barely any qualifications at all.”

Preston says the scandal was “the first time I’d been witness to a cause célèbre, where people seemed galvanized by the whole thing.” He adds, “The disbelief that someone who was supposedly the exemplar of high moral values had got involved in a ludicrous immoral plot to bump someone off—that intrigued me.”

The idea for a book had been at the back of Preston’s mind for years and was finally ignited when he discovered a copy of the 1981 memoir Cover-Up by Peter Bessell, the prosecution’s chief witness. It was “absolute dynamite,” he told a friend over supper a few days later. By coincidence, the friend had a family connection to the plot’s intended victim.

I myself still vividly remember hearing the verdict of what was by any measure a sensational trial at London’s Old Bailey in June 1979; I also remember the consternation of many when the accused was found not guilty of conspiracy to murder and of the separate charge of incitement to murder. In the court of public opinion, few judged Jeremy Thorpe, old Etonian former leader of the Liberal Party, to be innocent. At the very least, he was guilty of conspiracy to frighten and intimidate the man who claimed to have been seduced by him: the animal-loving Norman Scott, whose Great Dane, Rinka, was shot dead one rainy night on Dartmoor in a scene redolent of Sherlock Holmes. Even at the time, before respect in our public figures had taken a hammering, the judge’s summing up was considered wildly biased: he described Thorpe as a man of “hitherto unblemished reputation” while dismissing Scott as “a crook, a fraud, a sponger, a whiner, a parasite,” and he quickly became the subject of parody.

It was Rinka’s bloody end that propelled the whole story into the newspapers: her killer, airline pilot Andrew Newton, nicknamed Chickenbrain, was found guilty of possessing a firearm with intent to endanger life. Newton admitted to killing the dog but denied intending to kill Scott, who spoke from the witness box of his relationship with Thorpe and his belief that Newton had been hired to silence him. Scott was not believed, but Newton went to jail and, on his release, revealed a new story. “I Was Hired to Kill Scott,” ran a tabloid headline in October 1977.

Thorpe denied everything. But soon, a man named Peter Bessell—a former Liberal MP and friend of Thorpe who had kept Scott at bay while at the same time going along with Thorpe’s murderous plans—decided enough was enough. From his new home in California, he began, with some reservation, to tell the truth.

“Bessell seemed to me the lead figure in the drama because he’s the Judas character,” Preston says. “Why did he turn? One of the fascinating things about Thorpe was the degree to which he inspired extraordinary loyalty among perfectly sane people who were prepared to follow him through very thick and very thin.”

A story whose origins dated back to 1961 now began to unravel. Thorpe had taken Scott to stay at the home of Thorpe’s formidable monocle-wearing mother, supposedly en route to a business trip. There, in a house named Stonewalls, Scott was raped by Thorpe—who, the following morning, knocked politely on his door to ask how he’d like his eggs.

Thorpe was ruined by the trial, exiled from public life, and increasingly disabled by Parkinson’s. He died in 2014. His wife, the concert pianist Marion Stein, stood by him, refusing to entertain the notion that her husband was gay and dismissing the possibility that he could have plotted the murder of an alleged male sex partner.

“Homosexuality was at the core of it all, and it seems pretty amazing now that if you were a gay man you could be imprisoned,” Preston reflects. “Homosexuality was decriminalized in 1968, but the stigma remained and was perhaps even stronger postcriminalization.”

Thorpe, as an ambitious MP who described himself to Bessell as “80% homosexual,” had to marry—though his personal predilections were well-known among parliamentarians. Indeed, files detailing aspects of his scandalous conduct were passed to two home secretaries, but the omertà that existed among supposedly honorable members—many with secrets of their own—ensured nothing leaked. The press was deferential.

Scott, who these days lives a quiet life with sundry animals in North Devon, was happy to cooperate with Preston and lent him the unpublished diary he’d kept. “We talked and talked, and he never asked to see anything I wrote, never asked for money,” Preston said.

Bessell is long dead, but his son, Paul, was willing to help. So too was Tom Mangold, a distinguished TV journalist and a “Thorpe obsessive” who had interviewed Bessell. And Dominic Carman, son of George Carman, the defense attorney who annihilated the witnesses and forbade Thorpe to take the stand, fearing he’d condemn himself the minute he opened his mouth, provided his father’s case notes.

“All the protagonists were inadequate and to some degree leading secret lives,” Preston says. “Thorpe plainly was. Bessell was ostensibly a rather responsible member of society, but he had an incredibly rackety private life, in terms of his love life and his business affairs, and he topped it all by being a lay preacher. Carman, a leading Queen’s Counsel, a great establishment figure purportedly of great moral rectitude, was threatening to stick carving knives into his wife.”

The case made Carman’s name, and he went on to win some of the biggest criminal and libel cases of the late 20th century, despite an utterly chaotic, alcohol-fuelled private life. “I think his behavior was pretty extreme by anyone’s standards,” Preston says. “Dominic was as interesting as the notes. He’d been with his father throughout the trial, and George would rehearse in front of him. It was Dominic who suggested that in his summing up he go to the jury one by one, saying theirs were the most precious vote in Thorpe’s career. It’s a very effective piece of theater, memorable for the jury. The curious subtext was that it was the closest he’d ever been to his father.”

Thorpe, an MP on the right side of so many political issues, emerges as something of a tragic figure, a victim of his time who panicked and then lost his moral compass. A jury of readers can now reach their own verdict. That the central figure is “an obscure British politician,” now dead, matters not a jot, for A Very English Scandal makes for a splendid whodunit.

Liz Thomson is an author and journalist who has spent 30 years chronicling the international book trade.