Turquoise waters, white sand beaches, leafy palms blowing in the Caribbean breeze… the picture painted by Penny Farmer’s brother, Chris, in his messages home was of paradise, a world away from the mundane life he’d left behind.

Penny was just 17 when her ‘cool older brother’, 25 and recently qualified as a ­doctor, decided to go travelling with his law graduate girlfriend, Peta Frampton, 24.

The young couple left the quiet suburb of Chorlton-cum-Hardy, Manchester, in December 1977, spending six months in Australia before making their way to the Pacific Islands, the west coast of America and on to Belize.

Stuck at home with her parents, Penny couldn’t have been more envious.

‘Chris was a real spirited adventurer — he wanted to eke out the very last bits of life,’ she says. ‘I was very much in thrall to him. I think we all felt as if our lives were boring in comparison to his. For me as a teenager, it all seemed very exciting and exotic — he and Peta were travelling on a whim, always looking for the next new experience.’

Speaking to the Mail from her home in Oxfordshire, Penny, now 62, recalls how Chris used to post cassette tapes filled with updates and witty observations, all in his wry Mancunian drawl.

In July 1978, however, the tapes stopped coming. So, too, did Peta’s letters, which she had posted every fortnight from ­thousands of miles away, without fail, to her parents back in Chorlton.

Lives cut short: The last known  picture of Peta and boyfriend Chris

Spirited adventurer: Chris on his fateful trip with Boston’s sons Vince and Russell

Chilling secret: Though Duane Boston seemed a happy-go-lucky wayfarer, he was a prolific burglar who had dragged his children to Belize to escape rape charges in California.

The young lovers had, it seemed, disappeared into thin air.

It would be another six months before their devastated families, by then fearing the worst, would discover the awful truth.

Chris and Peta had been brutally beaten, tortured and murdered, thrown while still conscious into the sea off the coast of ­Guatemala, with their hands and feet tied and plastic bags over their heads. Their bodies had been tied to engine parts so they would sink. Fishermen found them 200 metres off shore a few weeks later.

This being the pre-internet age, with Interpol reliant on letters, phone calls and manual cross-referencing, there was no way of identifying the bodies, so they were simply buried in a local cemetery in Guatemala under plain white crosses.

With the families growing more anxious, Chris’s father Charles hired a private detective in Belize. Six months later, in January 1979, there was a breakthrough when he tracked down a Catholic priest, living in Guatemala, who recalled the bodies of the ­unidentified European couple found in the water the previous summer.

The news hit the families hard. They knew it had to be them. Who else could it be, in such a remote area? There followed, says Chris’s mother Audrey, now 99, a period of ‘awful, dreadful, black despair’ as they waited for the bodies to be exhumed and dental records to be flown ­thousands of miles across the globe to be cross-checked.

‘I was at work when the police rang up and said it was them,’ she recalls. ‘I can remember driving home and shouting to the empty car: “My son has been murdered. My son has been murdered”.’

But their nightmare wasn’t over yet. For the next 37 years, Audrey, Charles and Penny would exhaust every avenue to find out what had happened. Why had these two fun-loving, twentysomethings been murdered in such horrific circumstances — and who could have committed such a senseless act?

It took a ‘flash of inspiration’ and some dogged online detective skills from Penny to achieve what police on both sides of the Atlantic had failed to do: she managed to track down Duane Boston, the skipper of a sailing boat called the Justin B, on which Chris and Peta had been travelling before they disappeared.

One of Peta’s last letters home had described the weather-beaten American sailor, and his two sons, and their planned trip down the Caribbean coast to Honduras.

Boston, from Sacramento, California, was not only the last person to see the pair alive, but their suspected killer — and Penny found his profile on Facebook in 2015.

She also located his two sons, Vince and Russell, one of whom had a troubling confession to make: aged just 13, he had been on the boat with Chris and Peta and witnessed his father, an abusive drunk, murder them both.

Vince Boston’s story — along with the heartbreaking testimonies of Penny and Audrey — is told in a chilling three-part documentary, Dead In The Water, released on Prime Video yesterday.

Speaking for the first time on camera about his father’s heinous crime (one of many), Vince reveals he spent four decades living with the dark secret — and desperately trying to turn his father in.

‘It was a nightmare that you cannot wake up from,’ he says today. ‘I was thinking, this can’t be happening. My knees were shaking.’

He described in horrifying detail watching his father roll a still-­conscious Chris overboard, watching ‘bubbles rising up’ and hearing the terrified screams from Peta below deck. ‘Dad then took her weight, threw that over and then rolled her off the side. And then she just disappeared. He looked at his watch. A couple of minutes later, he said: “They’re dead.”’

Completely under his father’s control as a child, Vince, now 59, said he’d had nightmares for years about the incident. He didn’t dare ­discuss it with anyone, even his younger brother, who also saw it happen, for fear of retribution.

‘To watch your own father, someone that you trust, who’s supposed to be protecting you, murder two innocent people in front of your own eyes, you start to think: maybe we’re next. He looked at us and said: “You snitch, you die.”’ Later, as an adult freed from his father’s clutches, Vince’s repeated statements reporting what he’d seen to police fell on deaf ears.

Red tape, departmental politics and miscommunication between America and England seemed to be against him. Several times — because of the sheer, lucid horror of what he had seen that day — his claims were dismissed as a prank.

It wasn’t until he received a Facebook message from Penny, in October 2015, that he was finally taken seriously. All these years later, Penny and Audrey hold no grudge towards Vince. ‘I always thought the boys were the key to the story,’ says Audrey.

‘Initially I felt quite antagonistic towards them. But I’ve come to realise that they are as much ­victims of that man as we are.’

Penny agrees. ‘We have this unspoken tie to those two boys, because of what they witnessed. Our worlds have collided in the worst possible way. We don’t feel badly towards them. There was nothing they could have done. Chris and Peta were dead men walking the moment they stepped on that boat.’

The first the Farmer family heard of the Justin B, on which Boston ran ‘Robinson Crusoe’ sailing trips down the coast from Belize to Honduras, was from Chris’s tapes.

Audrey admits she ‘wasn’t even too sure where Central America was’ at the time. ‘Chris was very fond of sailing, which was a legacy of our holidays on Anglesey. The boat was very small, so they were more or less living on top of one another. But they’d been snorkelling and swimming and really enjoying life, so they were happy.’

She was, she admits, rather hoping her eldest child would come home soon. According to Peta’s letters, the couple planned to go on separately to Trinidad and New Orleans and then, his mother hoped, return to Manchester. ‘I remember I was missing him. The house felt very flat without him.’

The boat trip looked like another magical adventure to add to their collection of gap year memories.

A faded photo, unearthed years later, shows Chris in trunks, standing on the deck of the boat with a young Vince and Russell. All are tanned and salty-haired, and look relaxed and carefree. But aboard the Justin B, says Vince, things were not as they appeared. Though Duane Boston seemed a happy-go-lucky wayfarer, he was a prolific burglar, who had dragged his children to Belize to escape rape charges in California.

Even more chillingly, he was — and remains — the prime suspect in the murder of his third wife, Mary Lou, the boys’ mother, who disappeared in 1968 when Vince was three.

‘Dad would spend a lot of time getting drunk: he would sulk and then he would turn into this Jekyll and Hyde character,’ he remembers. ‘Nothing was good enough for him.

‘He would yell at Russ or me: “Why did you tie the knot that way?” or “Why did you pull the anchor up like that?” We were ­obligated to follow his every word. We didn’t have any radios, no way of communicating, so we were out there on our own.’

Chris, says Penny, wasn’t naive, but he was very trusting –— even of strangers. ‘You have to remember that this was the era of free love and hippies; they had this sense that everybody was kind and good,’ she explains.

According to Vince, it was an altercation between his father and his younger brother which sparked the acrimony between Boston and the travellers.

One night, Chris reprimanded Boston for hitting his son. The skipper, drunk on rum, lashed out to punch him, then fell overboard when Chris ducked out of his way.

The following day, seething and humiliated, Boston attacked Chris from behind with a club, fracturing his skull before stabbing him in the chest. He tied him up before going after Peta, who was below deck, stripping her naked and tying her up, too.

The following morning, according to Vince’s testimony, the ­monster claimed he was going to allow the pair — who were still alive but terrified — to leave, but instead tied machinery parts to them, put bags on their heads and pushed them overboard.

Back home, Chris and Peta’s families were growing increasingly concerned when the airmail ­envelopes stopped coming and they started making inquiries.

‘We later discovered from the Foreign and Commonwealth Office that Boston had left Livingston, in Guatemala, on July 19. He’d sold the boat and the next thing we knew was that he had gone back to Sacramento,’ says Penny. ‘Then, in October, they interviewed him and he said he’d put Chris and Peta ashore in Guatemala, across the bay from Livingston.

He also said that they were drug addicts, drug pushers, and that they had left the boat. We knew at that stage that he was lying. As a ­family, we couldn’t come to terms with anything, not knowing where they were. We were in limbo.’

Chris’s father Charles, who died in 2013, devoted himself to writing letters and making calls to foreign bureaus, police stations, newspapers and consulates.

‘It haunted him,’ Penny says. ‘We were all grieving in our own individual ways but one question was in all our heads: why? It just didn’t make sense, it didn’t add up.

‘It was like a bomb had exploded in our family and we were left to pick up the pieces.’

Distraught, she used to listen to her brother’s tapes over and over, determined never to forget his voice. Audrey wrote to Chris every birthday and Christmas for four years after he disappeared.

Time passed, however, and the trail for Boston went cold. In 1981, without the Farmer family’s knowledge, the UK investigation into Chris’s murder was closed.

Looking back, Penny says the case was ‘a mess’. It was a ‘flash of inspiration’ during a dog walk in 2015, that finally provided the breakthrough. She’d created an anonymous Facebook account to keep an eye on her three teenage children and used this to look for Boston, and also his now adult sons, Vince and Russell.

And there they were: ‘It was mind-blowing,’ she says. ‘I could not believe these people we had been searching for four decades were in front of me, on my computer screen. I told them I knew what had happened and that it was time to speak up.’

Vince gave a statement to detectives in Sacramento in October 2015. Using his testimony, plus old case files, which had been stored in a shed by the retired detective in charge of the case in the 1970s, they convinced police in Manchester and the U.S. to reopen the investigation. Boston was charged with murder in 2016.

Devastatingly, however, the case never got to court.

Just two weeks before Penny and Audrey were due to fly to America for a pre-trial hearing in April 2017, Boston, aged 75, died in custody.

He had heart and liver problems and, aware of the impending trial, had started refusing treatment — an act Penny describes as ‘the coward’s way out’.

‘To say it was another massive kick in the gut is an understatement,’ she says. ‘We could see and smell justice. It was almost within our grasp. He couldn’t have ­chosen any greater way to insult us.’

Audrey is stoical. ‘I just wanted to know what he looked like. I wanted to see the man in the flesh.’

Seven years later, the family are coming to terms with the injustice of it all. Penny has ­written a book about her brother and went to Guatemala, a painful but cathartic experience, in 2019 to visit his grave.

Every Christmas, the family raise a glass to Chris and Peta, whose parents have since passed away, and remember happier times: the young boy who always dreamed of being a doctor, who made his ­parents and sister so proud and who loved nothing more than being out on the open water, feeling the sea breeze on his face.

‘How unfair it seems that I’ve lived to 99 and Chris was killed at 25,’ says Audrey. ‘As a mother, I’d do anything to have given him some of the years I’ve had.

‘I know he would have gone on to do something wonderful.’

• Dead In The Water is on Prime Video, based on the book of the same name by Penny Farmer.