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Female lead was the adopted daughter of Han family, she was adopted by (adopted) father's Jin-Wook's previous wife. In the first timeline she married See-Hyuk who never loved her, the marriage was decided by their parents but she loved him cause he was kind to her. See-Hyuk has his own plans as Yuri was a candidate for marriage for much better people and she was so given more freedom cause she was loved by everyone. So the scum decided to marry FL and in that way he could be around FL.
FL regressed back and broke up with scum ex, she decided to have a contract marriage with ML who was quite wealthy and you know...CEO of I don't remember which firm..so back to story, Yuri was his potential marriage candidate but our FL proposed to him cause he just wanted someone to be his wife and she wanted someone whom she won't love and someone who's powerful enough and could help her in revenge.
As the story progressed, they fell in love and decided to be a real couple.
FL had her revenge, so the plot twist here is she was actually the biological child of Han Jin-Wook, her adopted father. Han Yuri was the outsider. Han Yuri's mother Lee Jung-Hye knew about it but lied to Jin-Wook who treated his own daughter as outsider for years while treated his fake daughter as apple of his eyes.
"My mother died and my father was him, I don't know why I was abandoned or how I was adopted but one thing is for sure. I checked it out"
So yeah, she did a background check and find something fishy, so she decided to check the DNA and the results came. I think her adopted mother knew that she's her husband's biological child. I quite don't remember this part.
Of course the selfish mother tried to convince her husband and he too believed her but at the end all the truth was in front of everyone.
ML's psycho brother who wanted the power tried to kill FL and had set up a bomb in her car but luckily she was saved and the brother was arrested.
She had twin sons and a daughter with ML and they all lived happily ever after
From user: @cookie_monster007

Spoilers about Han Yuri (Fl's sist- I mean man stealer)
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• This dumb creep was given everything when she was taken in by her (so called biological father). She hated FL and thought that she took her place. Before taking in by her so called father, her maternal grandmother took care of her and she was often alone. Her mother used to come late at him after work (work right? heh what was that, lying and conspiring). So when she saw FL living in luxury and having friends and all, she thought that FL took her place all these years and decided to destroy everything FL loved and severe her relationship with everyone who was fond of her.
• Han Yuri was obsessed with being love and her only motivation to live life was to isolate FL as much she can. So she intentionally spread rumours about her and her being adopted which made FL the outcast. Han Yuri's friend often bullied FL which included locking her up when it was Han Yuri's birthday so she can't be the part of celebration.
• She supported her marriage with See-Hyuk cause she knew that he loved her and is marrying FL only to be around her. She wanted FL's life to be as miserable as it could be.
• When FL married ML, and started to live a happy life she did everything she can to shake her life apart only to se her plans shattered.
• She decided to marry ML's brother so she could destroy both of them as he was also aiming for ML's destruction. So they both joined hands and went on with their idiotic plan.
• Later when it was revealed that she her biological father was a middle class man and her mother fooled Han Jin-Wook. She lost her face and backing.
• Her mother who always led her to wrong oath tried he to conceive ML's brother's , her so called husband's child and although she tried to sent against her mother and also got slapped but finally settled with the situation cause she got no other choice.
• She gave birth to a child who looked nothing like her husband. As you might've read in the above spoiler that her husband tried to kill FL, so he got arrested and now she got nowhere to go. Her condition got worse and she had no money.
• She went back to the man who was once her father. He gave her some amount of money and said that he hope it's the last time. She screamed at him and said he raised her as his own daughter for 20 years and now he's turning his back against her. He told her that he can't keep he as she reminds him of her mother's betrayal and also for his daughter's sake whom he thought wasn't his.
• She took the money and decided to go to her last option, See-Hyuk. And about the baby who looked nothing like her husband's, that child was See-Hyuk's, I think they did something hanky panky around the time when she was revealed to be the fake daughter. She didn't thought much about the possibility of See-Hyuk being baby's father and thought that the baby is Seo-Jungwook's.
• At first she tried to trick Seo-Jungwook's family into taking in the child so she can inherit the wealth through the child but ofcourse unlike Han Jin-Wook they were not so dumb and took a paternity test which proved that her wrong (she actually believed that the child is Seo-Jungwook's though she had her doubts after seeing the child's face)
• She stepped into the hell she once hoped that FL would enter.

Spoilers about See-Hyuk (Fl's piece of shit ex)
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This scum got cuckolded by her (Han Yuri) and she married another man and betrayed him so she already went off his love list.
• His father's company suffered emmense amount of loss, I think his mother brought an expensive building or something when she thought he'll marry the rich daughter of Han Family which didn't happened, but yeah he almost went from riches to rags luckily managed to get a potential rich woman who could help him save his company but that plan also went drown the drain.
• The day he brought his love home, Yuri came in barging the house with baby and told him infornt of everyone that the baby is his. His other asked him if that's true but ofcourse what he could say. He stayed silent and was deserted by his fiancé, his future is sealed as bleak cause now there's no way to save the company and on top of that he had to deal with two additional people. Yuri and their baby.
It was now a hell for both of them, for him who was left with nothing and for her who went to the same abusive selfish family FL went to in the first timeline. Now she would live an unloved life until writer knows how long .

Spoilers about male lead
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Seo Do-Guk is the second son of Sogang Group. He's caught up in a succession rivalry with his elder brother who's after his life. He needed marriage partner cause according to his grandfather's will the succession requires marriage. (I can't give more details cause this part was hard to understand due to machine translation) Ofcourse over the time he fell in love with the Female Lead. Han Yuri and his elder brother Seo Jungwook tried to create trouble and almost killed FL but everything turned out to be good at last. His older brother was arrested and FL and him lived in peace with their children.

Spoiler (about her shitty dad)
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As far as I remember she doesnt forgive him and he still lives on with the guilt of being fooled by fl stepmom and bitch sister. I also remember that those mother-daughter duo also pushed him causing him to lose the ability to use both of his legs. I mean he deserved it because he's stupid since as I mentioned in the main story spoiler he still believed what the mother said that the bitch sister was his real daughter and not fl until the truth was infront of everyone.

CAN SOMEONE TELL ME WHERE I CAN READ THE OFFICIAL CHAPTERS

https://m.webtoons.com/en/romance/there-must-be-happy-endings/list?title_no=3065
Here. It just got serialized today so it doesn't have more chapters. But it will have more and you can support the author there too.
I SWEAR THIS PART IS SUPPOSED TO BE SERIOUS BUT IM LAUGHING AND WHEEZING HERE THINKING WHATS GONNA HAPPEN NEXT HAHSHAHAHAHAHHAHAHA