Nayeong, a 29-old-single mom, goes through the baby care with help of three best friends of her late husband. Nayeong, born with natural optimism and somewhat recklessness, just lost her husband and depends on three distinctive characters. Suhyeon is a competent fund manager and most successful among friends, but most narrow-minded and stingy. Gwanghui is a handsome cartoonist with good taste of fashion, but an ultimate cynic. Gyeongtae is a young and good-looking detective, but too shy to speak in front of girls. Three guys at first feel burdensome when Nayeong asks for support. As time goes by, however, they start to feel a deep emotion toward the baby and to Nayeong as well.
Yoo Jin : Song Na-yeong
Song Na-yeong is an irresistible widow dashing this way and rushing that. "I am happy as a woman? Why? 'Cause I can be a mom!" A bit strange and somewhat insecure, she makes men want to take care of her. She always is babbling and playful but after her husband's death, she feels lonely and empty without anyone she can talk to. Na-yeong wants to be a strong working mom and she happens on her talent to become a consultant of designing apartment buildings.
Jae Hee : Choi Kwang-hee
Choi Kwang-hee is a cartoonist who likes girls but doesn't like the idea of marriage and detests children. He is a handsome but cynical guy who is fashionable, looks lonely and it seems there's something about him. So he is popular among girls. However, he is full of strange ideas. He is a cynic who seeks love affairs but doesn't know true love.
Jo Hyeon-jae : Han Soo-hyeon
Han Soo-hyeon is a succeeding fund manager but skinflint who wants to be the top 1% richest man. He is most successful among his friends but still has a long way to go. Step1; Searching for a millionare's daughter. Step2; Marrying her. Step3; Having an adorable daughter baby. He never loses his money and plays hard to reach his goal. But that's all gone to hell. Though he ignores poor girls, he falls in love with Song Na-yeong who is penniless.
Sin Seong-rok : Nah-hwang Kyeong-tae
Nah-hwang Kyeong-tae is a man who is a powerful detective in solving cases but trembles in front of women. "I want to get married. I want to have a baby as well. No matter what they say I am the dad. And there's no reason not to be her husband." He is hansome, but naive and shy in front of girls. Even though he likes babies and is willing to have parental leave, Na-yeong's baby cringes with him around. He wants to introduce Na-yeong and her baby to his mother as soon as possible.
Seongmin and Nayeong have been married for three years. But they don’t have a baby yet. All Nayeong wants is to have cute kids and give them all the love that she didn’t get from her parents. The problem is that it’s like she didn’t marry one man, bur four. Three best friends of Seongmin(Suhyeon, Gwanghui, and Gyeongtae) always get in the way. They even gather together to celebrate Seongmin and Nayeong’s wedding anniversary every year. One day, Seongmin confesses to his friends that he has a sperm deficiency.
The three friends find that, when they were drunken, they wrote oaths saying that they would donate sperm to Seongmin. Reluctantly, Suhyeon, Gwanghui, and Gyeongtae go to a hospital to donate their sperm and agree to keep that a secret. But later the three come to regret what they have just done. They ask Seongmin not to use any of their sperm and Seongmin promises so. At last, Nayeong becomes pregnant and everybody congratulate the couple. Unfortunately, Seongmin is killed by traffic accident on his way home after meeting his father-in-law.
“So one of us is the father?” Suhyeon, Gwanghui, and Gyeongtae are at a loss after watching Seongmin’s video. They have a quarrel on whether to tell Nayeong the truth that it’s not Seongmin’s baby, but a result of their donated sperm. At last, the three visit Nayeong to stop her having the baby but fail to tell the truth. That’s because Nayeong tells them the baby, Haseon is the last gift from Seongmin and it’s the only tie to him. Undergoing financial difficulties, Nayeong tries to find a job and is hired to sell children’s books. However, carrying a bunch of books, she faints on the street.
The due date has passed but the baby isn’t coming out. Nayeong makes every effort to make it come out soon. Suhyeon, Gwanghui, and Gyeongtae also feel nervous waiting for the baby. After all, Nayeong feels pain and takes taxi to the hospital all alone. Gyeongtae arrives at the hospital first. However, as he enters the delivery room, he faints in front of Nayeong. Finally, Nayeong gives birth to a baby girl. The three men are amazed and feel something hot in their hearts. They go to see Seongmin’s tree and promise to take good care of Nayeong and the baby.
Nayeong is interviewed for a job and hired at a construction company. On her first day, she has to go business trip to the countryside. But she can’t tell the company that she has a baby because the company won’t hire married women. So she asks Gwanghui to watch over Haseon. Without Nayeong, Haseon keeps crying and the three men don’t know how to feed her and to change diapers. Moreover, Nayeong doesn’t answer her phone and nobody knows where she is. Suhyeon, Gwanghui, and Gyeongtae are wondering why Nayeong gives them her baby and is not in touch. They jump to conclusions either Nayeong will abandon Haseon and run away or commit suicide due to postnatal depression. Meanwhile, Nayeong works hard and Manager Jeong Chanyeong shows interest in her idea on the model home.
Suhyeon and Gyeongtae blame Haseon worrying that they might get fired because of her. Gwanghui struggles to stop them while the two angry men insist they give the baby back to Nayeong right away. They head to the model home where Nayeong works for. On their way, however, they come to realize Haseon has a fever and diarrhea. Rushing to an emergency room, they regret that they are bad. Nayeong exhibits her ability at work and grows intimate with Chanyeong. But she finds out later that Chanyeong has plagiarized her idea on the model house.
Nayeong hurts her ankle and Chanyeong drives her home. Suhyeon, Gwanghui, and Gyeongtae happen to see the scene and Nayeong reluctantly introduces them to Chanyeong as neighborhood brothers and asks them not to tell Chanyeong that she was married and have a baby. The three get mad, misunderstanding that Chanyeong is Nayeong’s boy friend and, therefore, make their mind not to look after Haseon anymore. Now that Haseon’s gone, the three feel empty and miss the baby. Creditors visit Nayeong’s place and seize her house claiming that Seongmin has consigned the loan and passed away not repaying the loan.
Nayeong lost her house and has nowhere to go. Suhyeon finds her staying in a motel and takes her to the three men’s place. He even gives up his room to Nayeong and Haseon. Being grateful, Nayeong begins doing household chores. But everything she touches turns into a disaster. Meanwhile, Chanyeong calls Nayeong and asks her to work together as a member of Research task force team at his father’s construction company. Nayeong decides to leave Haseon at a daycare center so that she can go to work. As soon as Kyeongtae hears the news that a daycare center was set ablaze, he rushes to the daycare center and picks up Haseon saying he’ll watch her at home.
Gyeongtae suggests Suhyeon and Gwanghui to raise Haseon by themselves, but the two are not willing to do so. However, they reluctantly accept the suggestion because Gyeongtae warns them that he’s going to reveal the fact that they donated the sperms. Nayeong is deeply touched by their suggestion. Meanwhile, Chanyeong gets to work with Nayeong on a same project; he gets a good impression seeing her working hard. Haseon is now about one-year-old, raised in love and good care. The three men are getting used to raising Haseon in their lives. Suhyeon, who is stingy and only cares about his girlfriend Seoyeon, accidentally looses Haseon while he is looking for low-cost powdered milk. In the end, he finds her and realizes his love for this baby girl. Seeing Haseon toddling, the three men feel paternal love and prepare a small gift to Nayeong. However, shortly after the happy moment, they find Haseon disappeared while they thought she was sleeping…
Haseon is found at an unexpected place, and the three men are busy to find ways to raise this active baby girl safely. Even Suhyeon, who has been keeping himself away from Haseon, starts to enjoy nurturing her. Gwanghui dismissed after he screws up with his manuscript because of Haseon. He gets into a depression but Nayeong helps him to come back to his normal day. At Nayeong’s office, Chanyeong praises Nayeong for her creative ideas. One day, Haseon finally learns how to speak. When she speaks the word, ‘Daddy’, the three are amazed and so touched that they are not thinking about the difficult past anymore. They prepare the celebration of Haseon’s first birthday. On the event day, however, Suhyeon says a word to Gyeongtae that sinks down the mood…
It’s getting harder and harder to hide the secret. Nayeong, without knowing anything, prepares gifts for the three men when she gets a bonus from her work. The gifts move their minds and now each of them has secrets sharing with Nayeong. For Gyeongtae, the emotion doesn’t end as a small ripple, but becomes a big wave in his heart. One day, he goes to Nayeong’s office to give her an umbrella, and sees her with Chanyeong standing close together inside the unlighted office. Gyeongtae gets frustrated and can’t get the image of Chanyeong and Nayeong out of his mind. Suhyeon also sees Chanyeong and Nayeong together, when he is on a date with Seoyeon, and doubts about their relationship. Suhyeon and Gyeongtae share what they’ve seen and end up with the conclusion that the two are having a deep relationship. The three men can’t fall asleep and wake up shouting ‘That can’t be happening!’
Nayeong and Chanyeong open their minds, share each other’s secrets, and become closer. The three men are anxious about Nayeong because she starts to come home late; they even follow Chanyeong and start fistfight. Under the pretense of Haseon, Gwanghui tries to find out Nayeong’s feeling for Chanyeong. Suhyeon is also uneasy with his feeling, although he is dating with Seoyeon. Gyeongtae even secretly search about Chanyeong’s background, and gets jealous of seeing Nayeong with Gwanghui. Having difficult time getting out of his imaginations, Gyeongtae decides to tell Nayeong about his feeling for her. Nayeong’s project gets the good achievement and the team of Chanyeong and Nayeong dines together. Nayeong comes home a bit drunk and Gyeongtae helps her with her walking. Now Gyeongtae is about to say what he has on his mind…
Due to Gyeongtae’s mistake, everything appears on the surface and even the parents hear the news as well. It’s too late to get everything back. Shocked and confused, Nayeong comes out from the three men’s place. The men want to dissuade her from leaving but none of them can say a word to her. After much consideration, Gwanghui’s mother secretly requests DNA test. Stunned Nayeong goes to see Seongmin’s tree but can’t comfort herself from any place. The three men are also confused with the absent of Haseon and Nayeong; their house is full of the traces of Haseon and the memories of Nayeong is all over their minds. Nayeong tries hard to move on. It is painful, but the three men know they shouldn’t meet Nayeong. Nontheless, they go to Nayeong’s place for the chance they could see Haseon.
Gyeongtae gets into an accident and Nayeong runs down to see him. Nayeong and Suhyeon pray for Gyeongtae while he is unconscious. After all, Gyeongtae awakens, and gladly sees Nayeong and Haseon. Gwanghui sees Nayeong and talks to her about the hard time he had for the while. Chanyeong shows his feeling for Nayeong, but she doesn’t think much of it. Suhyeon is about to marry Seoyeon, but finds himself not as happy as he thought. Seoyeon asks Suhyeon to introduce his friends but Suhyeon just gives an ambiguous answer. Suhyeon is trouble minded after seeing Nayeong with Gyeongtae. Gwanghui makes contract with a publication company with high earnest money. Gwanghui’s gift touches Nayeong’s heart and Gyeongtae, the jealousy, also gives her the hotel
Different from Gyeongtae who has shown his love straight forward, Gwanghui has always been temperate in his feeling. However, Gwanghui tells the friends about his feeling for Nayeong, and the relationship among the three men moves into a big shift. Meanwhile, Suhyeon, who is always slow and uncomfortable with showing his feeling, has a good time with Nayeong and gets confused with his feeling. Nayeong is uneasy with it as well. Suhyeon rushes his marriage to wipe out his emotion. Feeling even more entangled, the four come back to Seoul. However, the shocking truth is waiting for them to be revealed.
The four friends, Seongmin, Suhyeon, Gwanghui, and Gyeongtae, grew up together and shared all the memorable moments. They thought donating sperms would be nothing compared to their deep friendship. However, after Seongmin’s death, Haseon comes into their lives. Hiding the secret, they nurture the baby girl and start to love her with all of their hearts. Nayeong, Haseon’s mother, also comes into their minds slowly but deeply. After having the hard time, Nayeong chooses her way to go to keep Haseon’s happiness. The three men know that they shouldn’t dissuade her from leaving. It is heartbreaking but they have to let her go…