The 15th episode starts with Woo-Hyun wanting to move in with Sung-Yeol and his parents. Mrs. Ahn doesn’t want him to and the father tells Woo-Hyun that he can stay the night tonight. Sung-Yeol gets upset and runs out of the house and his dad follows and his dad has alcohol. When his dad isn’t looking Sung-Yeol takes a shot of the alcohol. Woo-Hyun stands outside Seul-Bi’s window and they say goodnight and he waits for Sung-Yeol. Sung-Yeol and his dad come home and Sung-Yeol is drunk and stumbling. Mrs. Ahn and her husband talk and he is upset that she would treat her own son like that and that she would do the same to his son. Woo-Hyun tucks Sung-Yeol in for night and he acts all cute when he is drunk.
In the morning the two of them are sleeping in the same bed. In class Jae-Suk reveals that Sung-Yeol lives with Mrs. Ahn and everyone is in an uproar and she tells everyone that he is her son. Sung-Yeol tells Jae-Suk to come with him to the roof. Ye-Na and Young-Eun get in a fight and Ye-Na splashes her with water.On the roof Sung-Yeol encourages Jae-Suk to keep talking and blabbing.
Da-Yeol tells Ye-Na to keep her phone and Ye-Na asks her about her singing. Eun-Young hears them and catches Ye-Na blackmailing. Ye-Na starts crying and Seul-Bi tries to comfort her. Jae-Suk asks Da-Yeol who those guys where and she tells him they were from a music agency and that he wrecked it because it was her shot to impress them. He asks her if she wants him to start a label for her and she tells him that she feels sorry for him because he has no dreams. Woo-Hyun has gone to the nurse because his shoulder hurts and that if it keeps hurting he should go get it scanned. On the way home Sung-Yeol stops Seul-Bi from going to work with Woo-Hyun but she makes up an excuse and leaves anyway. Mrs.Ahn and her husband are talking at home and she tells him she got a loan to pay for the restaurant and he gets upset at her for not talking to him about it and thinks she is hiding more things from him.
Sung-Yeol comes in and hears it and is upset at her for using money and she tells him it was the money that she had before she married his dad. He tells her that she should tell everyone at school Woo-Hyun is her son and she tells him to do that so that she can face Sung-Yeol at school more easily. Seul-Bi goes to the restaurant and helps out Joo-Ah the cafe girl who has started to work at the restaurant. Joo-Ah had trouble at her previous workplace today because the owner was skimping on her wages and their old teacher got the money to give to her. Jae-Suk meets Sung-Yeol at the gaming place and Jae-Suk ordered food and Seul-Bi brought it and apologized to Sung-Yeol for lying. Sung-Yeol asks Jae-Suk for a favor and they all go the restaurant together and eat lots.
In the end Sung-Yeol refuses to pay the bill because his mother is the owner of the store now. Seul-Bi goes back with Sung-Yeol and she tries to talk to him about not hurting Woo-Hyun. Sung-Yeol sees Seul-Bi’s wrist glowing and watches it. At home he draws out the symbol from her wrist and the symbol glows on the paper.
Sung-Yeol’s dad talks to Woo-Hyun and Woo-Hyun tells him that he will try to pay him back but he needs time and will pretend he doesn’t know until then. They talk about how awkward it is and how hard it is for Sung-Yeol and Wo-Hyun.
Woo-Hyun and Seul-Bi go to the playground and talk about how she will be there for him. Sung-Yeol comes and they make a bet that if Sung-Yeol wins Woo-Hyun has to tell him Seul-Bi’s secret and if Woo-Hyun wins Sung-Yeol can’t tell everyone that he is Mrs. Ahn’s son. The bet is made on doing pull-ups and it is hard to do it with Woo-Hyun’s sore shoulder but he wins.
At night Woo-Hyun is in a lot of pain and looks for the pain reliever and Mrs. Ahn comes out and finds him and gets him the medication. Sung-Yeol at night goes into Seul-Bi’s room to look at her wrist and when he gets a look she wakes up and he puts his hand over her mouth and then leaves.
In the morning Mrs. Ahn stayed with Woo-Hyun all night and Sung-Yeol comes out and finds them and tells Seul-Bi he will make breakfast.
At breakfast Woo-Hyun notices the soup has shrimp in it and Mrs. Ahn takes his bowl and she eats it, Sung-Yeol yells at her to stop after a couple bites. Sung-Yeol’s dad scolds him for doing such a thing and asks him to stop for his sake. Woo-Hyun goes to see Mrs. Ahn in bed and she tells him she is sorry for just leaving him with an allergy. She tells him about when he was six and was crossing a busy street to visit grandma and spanked him hard and that her life was hard with a divorce and raising him alone was hard. That he understood and stopped crying and thought her raising him was a bad thing and he tells her that sounds like an excuse and she agrees. She acknowledges that it must have hurt and that is why she should have taken it for the both of them but has realized it too late and that she really is a terrible mother.
Seul-Bi and Sung-Yeol are at the park and she tells him to stop hurting Woo-Hyun and he approaches her and grabs her wrist and asks what she is because of the glowing. He is about to ask if she is something but Woo-Hyun shuts him up and her wrist makes a bright glowing light and this ends episode 15.