Transcript for Husband in love triangle murder-suicide falls for new boss: Part 1
She had planted recording devices in his office. It was a listening device. It was sewn into the lining of your jacket? Correct. Reporter: It sounds like a spy thriller. She had taken my phone and had it cloned. Reporter: But it's a real life love story, turned tragic. It took five seconds and I screamed and ran to her. And I just said to her, "Baby, what have you done? What have you done?" Reporter: An affair. Reporter: That lead to a fatal encounter between a wife and the woman she thought would replace her. I come outside all up and down the street it's just police cars and ambulances all flashing their lights. Reporter: Mark Gerardot first met jennair cox when they were just teens. When she met mark I can remember her talking, this is the one. She was very excited, she would do anything for mark. Mark liked her a lot. He was excited for us to meet her. When he's in, he's in. He was in love and this was the one. In 1993, said "I do." She was just a stunning bride. I'm the bride, I'm jennair. We were happy. And I couldn't imagine not being with her for the rest of my life. Reporter: They settled down in South Carolina. What would you say the biggest stress on your marriage was? I think, you know, money was always a concern. Reporter: The couple had no children. Both had worked in marketing. When jennair lost her job she lost a lot. She lost her pride and she couldn't find another job. And so that's when I found the opportunity for creative director at the university of Delaware. Reporter: Mark emailed the woman in charge of hiring. Meredith Chapman, who was 15 years younger than him. She was so articulate and accomplished to be in the position she was in. I was dumb founded. Reporter: Mark ultimately got the job and moved to Delaware. Jennair stayed back to lease our home and so I moved ahead 45 days. In those first few days and weeks where jennair was still back in South Carolina how would you describe life on the job with Meredith? It was jaw-dropping to see her command a room. Reporter: She had an impressive resume at the age of 32. She had ran for state senate in Delaware. Today I am announcing my candidacy for the Delaware state senate for district eight. And had been named to Delaware's 40 under 40 for young achievers. And eventually, finally she said I think after the first two weeks, she said, "Let's have a drink." When you went for drinks with Meredith you poured your heart out to her. I did. I don't know why, but it was so easy to talk to her. I didn't know it was happening. And I was confused by the feelings I was having, to be honest until I think I it became just a little bit more forward where we shared a kiss. And I immediately felt awful. And I said we can't do this. I can't do this to her. Meaning that you couldn't do that to jennair. I couldn't do that to jennair. Reporter: But mark couldn't deny his feelings for Meredith. It's hard to say how I felt. I felt enthralled. It was such a connection to shut it down just didn't feel right. And I had to at least find out what it was that was that drew us together so much. Mark and Meredith find themselves embarking on a whirlwind romance. Meredith was also married. They'd been married for nine years, and the last three had been a show. Did your relationship with Meredith reveal the problems in your marriage? Here comes Meredith and she's just telling me that I'm a wonderful man. I'd never heard that from jennair's lips ever. Here's this woman who I think is amazing saying that she thinks I'm a wonderful man. She'd only known you for a few weeks though? Fair point. She'd only known me for a couple of weeks It was a very powerful feeling. And I was overwhelmed by it all. I didn't know what to do with the feelings that I was feeling. They were new, unexpected. At the same time, they were, it was addictive as well. We'd gotten into a pattern that was over and over again, the same thing. And when I got to see that life can be different, there's something else, I questioned everything. Reporter: By new year's 2018, jennair had just moved to Delaware to join mark. She began to notice something was off. She suspected it had something to do with another woman. She said specifically, "It's Meredith, isn't it?" We had talked about Meredith before in the context of her professional decorum and I ooh-ed and aah-ed about Meredith. It wasn't too far of a leap for her to get there. And I denied it. Reporter: But jennair isn't buying it and starts quizzing about Meredith. With information she couldn't have any way of knowing and mark can't figure out how. Unbeknownst to him his every word was being recorded. I've felt more joy, more excitement this fall than I ever did before. I want to see you again today. I really want to, I kind of need to. Where are you going to be? Love you. Thanks. Love you too. Bye. Reporter: But by Valentine's day, in less than two months, the situation came to a breaking point. At that time jennair disclosed that she had taken my phone and had it cloned. I finally just told her, "I'm in love with Meredith. I can't deny it anymore." And how did she react? I think she was a bit taken back that I was so bluntly honest. But to me, it was a relief to actually get it out in the open. To save your marriage? We were going to go to marriage counseling. We had different, I think, intentions. Mark is getting ready to head to the marriage counselor's office and throws his coat on and he feels something in the lining of the coat. And I reached inside my pocket and I felt what I thought was an antitheft device. I cut open with an x-acto blade the edge of the pocket and pulled out this device, and there was a blinking light. It was a listening device? It was a listening device. Reporter: Mark says jennair claims she's already told their boss at the university of Delaware and Meredith's husband about the affair. I told her, "I'm going to be filing a petition for divorce Reporter: Meredith and her husband are also in the process of divorcing. And both mark and Meredith start looking for new jobs. Meredith accepted a prestigious position at Villanova university as an assistant vice president. Reporter: Jennair on the other hand is looking ahead to what she feels is a life alone in a new city with no job or support system. Everybody says it's over. I'm not there yet, I can't accept it. Jennair was concerned about being tossed away. You don't find me appealing. You don't find me attractive. You don't want me anymore. You don't even like me. You are miserable. She lights your fire, she makes you feel young. She had huge fears about being left financially. I realize that this whole situation has caused you not to be able to get a job. I get that, I really do. You screwed the pooch, big time. On us, my future, your future, our future, our finances. Everything. You screwed the Pooch because you made an immoral, selfish choice. Reporter: During this time, mark says jennair started acting erratically. She was threatening to jump out the window? That was certainly the impression that I was given. Reporter: He says he encouraged her to reach out to family and friends and a psychiatrist, which she says she did. And I thought, "I'm doing all I can do." Reporter: According to mark she seemed to pull herself together and brace for the inevitable divorce. How was she acting during that time? Completely normal. Did it feel bizarre for it to seem so normal? It wasn't all sunshine and roses. There were moments of she would get upset and she would cry and she would break down. Reporter: But jennair's action were far from normal. When we come back -- I can't -- it's too painful for
This transcript has been automatically generated and may not be 100% accurate.