A new book details how a British woman solved the gruesome murder of her brother and his girlfriend decades earlier, the Daily Mail reports.

Penny Farmer writes her first-hand account of the case in the forthcoming book “Dead In The Water,” which is set to be released early next month.

Farmer was just 17 in July 1978 when the bodies of her brother, 25-year-old Christopher Farmer, and his girlfriend, 24-year-old Peta Frampton, were discovered by fishermen floating in the Caribbean near the Guatemalan coast.

The bodies were bound, weighed down with engine parts and there was evidence that the victims had been tortured.

The previous year, the couple had set out to explore the world, traveling from Australia to Los Angeles and Mexico to Belize.

Christopher called home while Peta wrote letters to her mother.

The last contact the couple had made with their families was via a letter Peta wrote dated June 28, 1977, in which she detailed how she and Christopher met an American, Silas Duane Boston, and agreed to sail the Caribbean with him and his two sons, Vince and Russell.

But after months of silence, the couple’s families in Britain grew concerned.

Farmer’s father wrote to the harbormaster in Belize City seeking records of Boston’s sailboat, named Justin B.

The harbormaster replied that Christopher and Peta had been on the sailboat when it departed, but there was no record of them returning.

Further investigation found that the couple had secured Honduran visas but had not entered the country.

In October, authorities learned that Boston had been located in California. He was evasive when first interviewed and asserted that Christopher and Peta made it back to Guatemala.

But his answers to questions caused only deeper concern, and so the couple’s parents went to police.

Diplomats also were wary, and the British Consul dispatched an official to interview Boston personally in December 1977. At the time, he appeared “calm and relaxed.”

But when questioned about the couple, Boston reportedly sat right up and began breathing heavily. His eyes widened.

He then slouched back in his chair and softly said that he thought the couple “would be back home by now.”

The formal investigation seemed to be going nowhere, but Christopher’s father, who worked for the BBC at the time, worked his relationship with the press to raise awareness of the situation and plead for information.

Dr. Tom Lane, an American, responded to the family and asked a friend from Belize, Alphonso de Pena, to investigate.

In January 1979, a priest living in Guatemala near the Belize border told de Pena that fishermen the previous year had discovered a dead couple in the water.

Family members paid to have the bodies exhumed and had dental records flown to Guatemala to identify them.

Meanwhile, Interpol reported that Boston’s third wife had gone missing in September 1968.

By May 1979, Manchester police asked law enforcement in Sacramento to interview Boston, but he had disappeared by then.

It would be 37 years before the case moved again.

In 2015, Penny Farmer found Boston’s oldest son, Vince, on Facebook. What she saw on his page shocked her: “My mother was killed at 23 with a gun.”

She later found Boston himself on Facebook and soon after contacted cold case investigators in the U.K.

That led to Farmer getting in touch with Amy Crosby, a detective with the Sacramento Police Department, who happened to be investigating the death of Boston’s ex-wife and mother to Vince and Russell.

Vince provided a statement to police – that his father had indeed killed his mother – but where she was buried was unknown.

There was more: Vince then told Crosby he also had witnessed his father kill Christopher and Peta.

Information would soon surface that Vince had attempted to alert law enforcement multiple times about the killings but to no avail.

When he was 16, Vince joined the Navy and told London police what had happened to Christopher and Peta. He even provided the couple’s full names, but he was told there were no records of the case.

After police had obtained statements from both Vince and Russell, police met with Farmer and her mother.

“How much would you like to know?” Officer Martin Bottomley asked.

“Everything,” her mother said.

According to the brothers, Boston had been on the run after being accused of statutory rape in Sacramento. They also said he’d get violent after imbibing on rum.

At some point on the boat, an inebriated Boston began hitting Russell. When Christopher stepped up to stop the assault, Boston attempted to punch him but instead fell into the water.

It was at this point when Boston began planning to kill the couple, Vince said.

The next day, Boston told Christopher to pull the anchor. He then snuck up behind him and began clubbing him in the head.

When Boston stabbed Christopher in the chest with a knife, the blade broke and Christopher exclaimed, “I give up!”

The next day, Boston told the couple he would drop them off by Livingston, but would strip them naked and tie their hands so they couldn’t report him to police.

But soon after, Boston instead put plastic bags over their heads, tied them to metal and then threw them overboard while fully conscious.

A hushed silence fell on the room after Bottomley finished reading the accounts.

On Dec. 1, 2016, prosecutors charged Boston with murdering Christopher and Peta. He died the following April while in police custody, his health poor from years as an alcoholic.

Police believe he could be responsible for multiple other murders over the course of 50 years.

Farmer wrote that Boston remained defiant until his death. Prison guards said he was “controlling with an evil glare right to the very end.”