this is "nightline." tonight, a fairy tale marriage. >> i'm the bride. >> turned deadly love triangle. >> i finally just told her, i'm in love with meredith. i can't deny it anymore. >> he swept away by a secret romance. she and her suspicions. plus, the revenge plot, shocking lengths the woman scorned went to. secret surveillance. >> she named my device and called meredith's device fore.

>> sewing devices into his clothes. all reaching a tipping point. >> i just said to her, baby, baby, what have you done? what have you done? >> "nightline" will be right back.

>> reporter: good evening. thanks for joining us. for jennair gerardot three was certainly a crowd. a wife who thought she had it all only to be betrayed by her

husband and brought to a breaking point sending a startling final member before committing the unthinkable. here is abc's amy robach. >> 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, oh 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 s 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 [ bleep ] 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 me to see you living your life happy. she's living my [ bleep ] life. hey. this is amazing. with moderate to severe ulcerative colitis, are you okay? even when i was there, i never knew when my symptoms would keep us apart. so i talked to my doctor about humira. i learned humira can help get, and keep uc under control when other medications haven't worked well enough. and it helps people achieve control that lasts. so you can experience few or no symptoms. humira can lower your ability to fight infections, including tuberculosis. serious, sometimes fatal infections and cancers, including lymphoma, have happened; as have blood, liver, and nervous system problems, serious allergic reactions, and new or worsening heart failure. before treatment, get tested for tb.

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ended his 24 year marriage, mark geradot thinks his wife jennair is coming to terms with their impending divorce. the two planned to meet over dinner to discuss it. >> when i got there it was so nice outside they had an outdoor patio and i said, "i'm going to be sitting outside." she said, "okay, great." jennair texted me and says, "i made a wrong turn. i'm going to be late." 10 minutes or so go by and she texted me back and said "i'm not coming. just go home." and she sent me a text. it was a photo of trash. there was a condom in the middle of that photo. i knew the context of it. i thought she was outside of meredith's house going through her trash. >> looking for a condom? >> looking for evidence of any kind that i had been there. and then she sent me a text that said, "you ruined my life." and then another text that said, "i hope you never find happiness." and a third text that said, "bye, mark." meanwhile, i'm already texting

meredith and she's not replying to me. i thought, okay. so jennair went over to her house. there was a confrontation. and so i paid the bill and rushed over to her house. so i walked to the back of the house and i peered in. >> reporter: what he saw shocked him to the bone >> i saw meredith face down on her kitchen floor. and i rapped on the door and yelled her name. and she wasn't acknowledging me. i went to her and i saw a fleck of blood on her calf. i just couldn't understand why there was blood. so i ran out onto the porch. >> i hear a commotion outside. and mark is there. he is freaking out. >> and i said call 911. >> i open the door i notice that in her left hand she still has her keys which i find odd. so i go around and i feel for a pulse on her body.

>> reporter: but meredith wasn't alone. >> and that's when i saw another figure laying there, and she was dressed all in black from head to toe. and i didn't recognize the face, and yet i did. >> it was jennair. and i just said to her, "baby, oh baby. what have you done? what have you done?" >> i actually heard the radio transmission. it came in as two deceased people in this community, there's not a lot of calls like that. >> at that moment, my heart just sank. >> at first i thought we were dealing with a double homicide. >> as i approached, the -- the front door had a closed in porch. there was an outer door, and then there was an inner door. that inner door had a broken out pane of glass. the one clos handle was missing.

somebody broke into the home and killed somebody? >> i don't know what i said to the police. >> he described the one woman as >> i was yelling at them to get in there and help them and there was some woman who was helping me finally just grabbed my hand and said they're gone. and that's where i completely lost it. finally, the police whisked me off to the police department. >> because you were the prime suspect. >> i was the prime suspect. >> the murder weapon found was a .357 caliber revolver. >> two rounds had been fired from the handgun. >> five rounds still in the weapon. >> the police said we have to ask this question, "did you shoot these two women?" and i literally said, "shot them? they were shot?" that's where i pieced it all together. but it still wasn't real. >> it was determined that jennair had committed a burglary and broken into meredith's home. >> where she lay and wait for

her to return from work. >> and had murdered her before taking her own life. >> is there any way to describe what you were feeling? >> nightmare. my world was gone. >> we are learning more about the murder/suicide. by all accounts it appears to be a jealous attack. it became very clear to me when the news broke, that i had seemed to be in many people's eyes public enemy number one. >> mark is in a state of shock and grief. shunned by his friends he channels all that energy into an obsession to find out what was going on with his wife. >> did you ever think that jennair was capable of physically harming? >> never. she had never done anything violent. >> reporter: but the police uncover evidence that jennair

had been meticulously surveilling her husband. >> i had to know myself, how she pulled this off why she did what she did. >> reporter: mark starts pouring through jennair's bank statements, phone records, computer backup. turns out the listening devices mark found earlier were just the beginning. >> she had told me at some point that all the audio recordings had been destroyed. but then to see them all, hundreds of hours of recordings. then she actually transcribed every word of it, in notebooks. there were twelve notebooks full of transcripts that she had written. >> reporter: mark finds transactions for a lock-picking kit he believes she used to break into his office. computer hacking software. a sophisticated gps tracking system she personally installed on both mark and meredith's cars. >> so, on the actual gps maps, there were two icons, one showing my car and one showing meredith's car. she named my device lying jerk,e whore. >> reporter: she had been watching them for weeks. tracking those two icons

wherever they went. >> there were over 400 images of private conversations that meredith and i had had via snapchat, and my best guess as to how she gained access to my phone was in the middle of the night. i was sleeping on the couch. she put my thumb on the reader. >> reporter: mark says he also found the receipt for the murder weapon and the box it came in. >> she bought the gun on march 20. close to five weeks before she used it. it's unbelievable to think we were headed towards what i hoped to be a civil end. >> reporter: when we come back, mark tries to make sense of the unthinkable. >> i could have stopped the relationship, handled it completely differently. i broke her heart and my regret comes back to making her feel like she had no other choice. there's so much scent in new gain scent blast detergent... ...you'll either love it or...

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"nightline" continues. here again, amy robach. there are days that i have to convince myself that it happened. there are days where i see jennair. i see some woman who has her build and her hair and i think, "ah, it was all a ruse." i see meredith and so obviously i'm not over the shock. >> reporter: mark says he's trying to process this tragedy by writing a book. >> have you forgiven yourself? >> i'm still working on that.

i'm still working on that. >> have you forgiven jennair? >> i have forgiven jennair for what she did to me, and i have yet to forgive her for what she did to meredith. she took someone else's life. it's unforgivable. i do not forgive her for that. no. >> reporter: our thanks to amy for that story. and a programming reminder, abc news and univision will host the upcoming democratic primary inbate n. at'sightline. have a great weekend. good night, america.

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