"This is the most beautiful place in the whole world to hear the most terrifying thing you've ever heard," Joannah told ABC News' "20/20" in an exclusive interview.
Joannah, who asked that ABC News not use her last name, said Konrath had talked about murdering Ana Konrath, his ex-wife and the mother of their three children, once before on the trip, so when he brought it up again, she reached for her cell phone and started secretly recording him. Ana Konrath declined to comment for this report.
"He had had a few drinks and we're on the top floor of the hotel… and he starts talking about Ana ... and how he's planned this for over a year," Joannah said. "I decide I'm going to record this because I mean, I don't know… I didn't want him to kill the mother of his children."
For the next 23 minutes, Joannah recorded Konrath as he laid out in great detail what sounded to her like a chilling murder plot, and how he said he planned to make it look like a suicide.
"One choice is to do it in the bedroom… if for some reason the kids woke up, there's lots of ways to get out," Konrath is heard saying on Joannah's recording. "Take an unmarked gun that has no prints with bullets already planted in her house, which they are… because she shoots herself in the head, she kills herself."
"If I'm not on-call, I [can't] bring my cell phone because that's traceable," Konrath continues. "Suspicion would fall immediately on me, the ex-husband… she has a million-dollar life insurance policy. I'm the beneficiary."
Konrath went on to explain how he already had been very careful with the gun he wanted to use as the murder weapon.
"I never touched it with my hands," he says. "Even when I took it to a shooting range, I wore latex gloves… after I do that… I put her prints on the bullets, close it, put her prints on the handle."
He even went on to lay out the challenges of killing his ex-wife.
"One thing I don't know is, is she going to ------- slump down and die right there, that would be ideal, but maybe she'll writhe around and start making noise," Konrath says. "I've done a lot of research on that, that's why I switched my phone by the way. I used it to research it and then, you know, nothing is traceable."
Konrath also had calculated the risks of killing his ex at her home.
"I already drove over there twice … ready to go, and both times something happened," he said, adding that he had already "pre-arranged messages" to her family informing them of Ana's "suicide."
Joannah, a nurse who is also divorced with children, met Konrath through the dating website, OurTime.com. They moved in together after dating for four months and had been dating for almost a year when he took her on vacation to Puerto Rico. She had hoped they would get married, and said they had looked at wedding rings and had already purchased a wedding dress.
But after Konrath revealed what appeared to be a violent plot that he had been hatching to kill his ex-wife, Joannah said she was upset, and secretly plans to race back to Indiana without him.
"He wasn't just blowing off steam," she said. "He talked about buying a gun and hiding it and where he was going to do it and how he was going to do it and how her body was going to fall and he had done research."
Joannah said Konrath said he confided in her because she said he wanted her to tell the police he was with her so that she "could be his alibi."
Then Joannah started asking Konrath questions that seemed to catch him off guard. Joannah is heard on the recording saying to Konrath that he needed to think about money, and to "set everybody up." Then she asked him if they could get married before he kills his ex.
Joannah told "20/20" she was only focusing on her well-being and wanted to make sure he would look after her financially if he decided to go through with this alleged plot. But, Joannah claims she had always planned to turn the recording over to police, which she did.
In an interview with "20/20" from jail, Konrath said all that talk about killing his ex-wife was just a "dark fantasy" and "a lot of it is drunken BS."
"We were right on the beach… it's vacation, so I was drinking a fair amount," Konrath said in an exclusive interview. "Then I just told her about this kind of, I guess, dark fantasy that I had about killing Ana."
"I have never seriously hurt anybody," he continued. "And I don’t have any desire to kill somebody in real life. So if you talk to me about would I want to kill Ana when I am sober, I would say, 'No, I don’t want to.' And not only that, but I am not gonna take a chance on my life being wasted by an attempt."
Konrath is currently serving a 10-year prison sentence in Indiana, but it is not because he was convicted of attempted murder. It's for an entirely different reason. Watch ABC News' "20/20" HERE to find out why.