Dan Dan grows up being told that her birth mom is dead but still becomes a hard-working young woman. One morning, she overhears her stepmom and stepbrother talking about something that sounds like trouble for the family. Meanwhile, Young Guk is a single father of three kids after losing his wife two years ago. He asks a friend to help him find a live-in tutor.

Dan Dan takes Young Guk for a handsome psychopathic criminal, like in the movies, and she hits him in the head with her water bottle. Young Guk ends up with a gash on his forehead and is baffled by Dan Dan's plea for forgiveness. Meanwhile, Guk Hee suggests Dan Dan go in for an interview to become the live-in tutor at Young Guk's house.

Dan Dan gets hired to work as the live-in tutor at Young Guk's house, thanks to the help of Sa Ra. However, she gets into trouble on her first night on the job. Meanwhile, Sa Ra gets invited to a fancy restaurant by Young Guk and begins to get her hopes up. As Dan Dan tries to get used to living and working in Young Guk's household, she runs into an unexpected familiar face.

Dan Dan and Soo Cheol realize that they work in the same household and worry that their employer might think they are up to something suspicious. Yeon Sil tries to persuade Dan Dan to leave her position, but Dan Dan refuses. Meanwhile, Young Guk decides to let his late wife rest in peace, and he has her things cleared away.

Young Guk's strict nature with his kids makes his relationship with Se Jong worse than ever. Young Guk enlists Dan Dan's help to resolve the matter. Although they bicker about how to handle the kids, Dan Dan's advice makes the kids grow brighter by the day. Meanwhile, a mysterious man appears before Sa Ra, which makes Ki Ja suspicious about her daughter's past.

Young Guk and Jae Ni fight when he mistakes Jae Ni's new friend, who happens to be a boy, as her boyfriend. Dan Dan helps the father and daughter make up, and Young Guk feels grateful. Meanwhile, Yeon Sil's mom, brother, and niece return from working abroad for five years. She asks them to come to the annex at Young Guk's house and trouble ensues.

Young Guk's family treats Soo Chul as if he were a parasite and a fraud, and they kick him out. As Dan Dan watches her dad endure such humiliation, she decides she must leave her job as well. The children see how sad Dan Dan is, and they come up with a plan to cheer her up. Meanwhile, Anna begins looking for a way to get back in touch with her daughter.

Soo Chul finds out that Dan Dan got an advance on her salary and gave it to Yeon Sil. He becomes upset and goes to see Dan Dan to return the money. However, he is only met with more disappointment when Dan Dan asks to continue living as strangers. Meanwhile, Anna decides to stay at Young Guk's for a while to be close to Dan Dan.

Dan Dan is upset that she has to lie about going to see her dad in the hospital and ends up blurting out the truth to Young Guk. When Young Guk hears of her relationship to Soo Chul, he becomes angry that she was untruthful and fires her. Sa Ra is glad to see Dan Dan go and takes this chance to get on Dae Ran's good side. Meanwhile, the kids miss Dan Dan dearly.

When Dan Dan returns to help console Se Jong and continue working at Young Guk's house, Sa Ra feels threatened by her. Se Ryeon gets drunk and barges into Dae Bum's house, asking him to take her back. The two families end up clashing once again. Meanwhile, Dan Dan joins Young Guk and the kids on a trip to their vacation home for Se Chan's birthday.

Young Guk finds Dan Dan in the woods with a sprained ankle. Sa Ra becomes angry and jealous when she witnesses Young Guk carrying Dan Dan on his back. Meanwhile, Dal Rae sees a photo of Mi Sook and her younger brother and is reminded of Soo Chul's childhood photo. Later on, Young Guk invites Dan Dan out to a restaurant.

Dan Dan and Young Guk realize that they have a past connection. When Sa Ra sees how close Dan Dan and Young Guk are getting, she is overcome with jealousy. Meanwhile, Se Ryeon meets the man Dae Ran wants her to marry. While on her date, Se Ryeon runs into Dae Bum in his food truck. Finally, Dae Ran and Sa Ra scheme to have Dan Dan thrown out.

Dan Dan confesses her feelings for Young Guk in a drunken state which sends her into a frenzy the next morning. It makes her even more nervous to face Young Guk, and Young Guk also starts to notice Dan Dan in a different way than before. Meanwhile, Dae Ran and Sa Ra decide to lie about the dementia test results and continue to harass Dan Dan.

Dan Dan gives Young Guk time to make up his mind about her until the next day at midnight. Young Guk contemplates long and hard about what to do. Meanwhile, Dae Bum realizes that Se Ryeon is about to get married to a two-timer and tries to stop her, leaving Se Ryeon feeling confused. Later on, Sa Ra witnesses Dan Dan with Soo Chul and tries to use it to get Dan Dan kicked out.

Sa Ra confesses her feelings for Young Guk, making him decide that they cannot work in the same house anymore. As Sa Ra resigns and leaves Young Guk's house, Dan Dan and Young Guk's secret relationship starts to get more serious. Meanwhile, Sa Ra calls a driver to take her home after she has too much to drink, and Gun shows up for the job.

Young Guk confesses to Dan Dan that he couldn't tell Anna about their relationship, and the two agree to take things slowly. However, Young Guk continues to worry that Dan Dan might get hurt by other people's comments about their relationship. Meanwhile, Kyung Seok's girlfriend shows up just as Se Ryeon's wedding is about to begin.

Dan Dan returns home alone after waiting to meet Young Guk. She is shocked to hear that he is still not back from his hiking trip. Young Guk's family and friends call the police and desperately search for him, but Dae Ran imagines making Se Ryeon the acting chairman if Young Guk isn't found soon. Later on, Young Guk comes home but in a shocking state.

Sa Ra and Dae Ran decide to lie to Young Guk and tell him that he and Sa Ra were engaged to be married. As Young Guk feels confused, Dan Dan thinks he was two-timing her. She decides to quit her job and tells the children goodbye. However, a small part of Young Guk's memory comes back, making Sa Ra more anxious than ever.

Jealousy overcomes Young Guk after seeing Dan Dan and Hyun Bin affectionately together. Using coincidence as an excuse, he tries to spend as much time with her as he can. Sa Ra attempts to get close to the 22-year-old Young Guk, but it's not as easy as she thought it would, and she becomes irritable. Meanwhile, Young Guk participates in Se Jong's sports day.

Young Guk feels guilty about liking another woman when he is just about to get engaged. He tries to make himself like Sa Ra but finds it tough to get Dan Dan off his mind. Meanwhile, Sa Ra is appalled after learning the newly dispatched diver is Cha Gun and will work at Young Guk's house.

As the day of her engagement draws near, Sa Ra feels anxious about Gun's presence in Young Guk's house. She decides she must do something about it and asks to see him. Meanwhile, Young Guk can't hide his feelings for Dan Dan any longer and confesses his feelings to her. However, Dan Dan's reaction is not what he expected.

Young Guk continues to get bits and pieces of his memory back. He wonders why those memories include Dan Dan and asks her about it, but Dan Dan refuses to admit that there was ever anything between them. Meanwhile, Sa Ra grows more anxious as Young Guk seems close to regaining all of his memories. She decides to do something before he does.

Young Guk regains his memories, throwing Sa Ra and Dae Ran into a frenzy. Dae Ran begs Sa Ra not to tell Young Guk about her involvement in their scheme, but Sa Ra declares she will not give up on marrying Young Guk. Meanwhile, Se Chan and Se Jong decide that they would rather have Dan Dan as their new mom than Sa Ra. They start to play Cupid between Young Guk and Dan Dan.

Anna goes to Yeon Sil and asks her to persuade Soo Chul to come and work in the US with her. After hearing the salary Soo Chul would get if he took the offer, Yeon Sil pushes Soo Chul to accept. Meanwhile, the conflict between Young Guk's kids and Sa Ra worsens. Dan Dan decides not to hold her feelings back and to stay by Young Guk's side.

Yeon Sil and Anna finally convince Soo Chul to go work in the US, but now Dan Dan refuses to go. As Young Guk tries to make Dan Dan leave with Anna, Se Chan and Se Jong come up with a plan to make Young Guk and Dan Dan grow closer once again. However, when the boys' plan goes up in smoke, Young Guk discovers how Sa Ra has been treating the kids behind his back.