While selecting the winners for a social media event, Jang Mi-ho gets in touch with Yu-jin, a bad relationship from her past. Meanwhile, Yu-jin is cornered by other moms who suspect her of lying for the main role in a play. One day, Mi-ho shows up in front of Yu-jin and fearing that her past will be discovered, Yu-jin gives money to Mi-ho to silence her.

Because of an incident during the play, the event is suspended, and Yu-jin and Ji-ye argue over whose fault it is. Yu-jin accepts Mi-ho's offer to shoot a commercial, but the shoot is canceled when Ha-yul gets hurt on set. The emotional rift between the two deepens. Mi-ho goes to Yu-jin's house to apologize, but is kicked out. The next day, Mi-ho hears shocking news about Yu-jin...

Rumors spread at work that Mi-ho is directly involved in Yu-jin's death. Meanwhile, Mi-ho goes to the aquarium with Ji-yul and Ha-yul. While returning to work with a complicated mind, she gets a call that the girls have disappeared. It turns out that they got lost while going to see Mi-ho. Mi-ho decides to take time off work to care for Ji-yul and Ha-yul, and moves into ‘High Prestige.’

Mi-ho is clumsy in everything about childrearing, and she doesn't fit in properly with other moms. Na-young shows hostility towards Mi-ho and Mi-ho learns from Ji-ye that Yu-jin had a grudge against Na-young in her lifetime. After Mi-ho witnesses Na-young's serious delusional jealousy that doubts her husband's conduct, she finds a picture of Tae-ho's affair in Yu-jin's house.

Mi-ho takes the pictures of Tae-ho’s affair to the police but fails to see the detective in charge. After an argument with Mi-ho, Na-young goes to Jung-ah to kick Mi-ho out, but Jung-ah doesn’t comply. When Mi-ho learns Tae-ho is having an affair with A-ra, she decides to apologize to Na-young. Meanwhile, after being humiliated by the other moms over lunch, Na-young decides to hang herself.

Mi-ho finds Na-young just in time and saves her life. When words get out about Na-young’s suicide attempt, Jung-ah blames it on Mi-ho and looks into her past. Meanwhile, Mi-ho gives up on finding Yu-jin’s killer and returns to work. But feeling guilty, she decides to look after the kids and find the truth about Yu-jin’s death. One day, she receives an anonymous text about the USB.

Mi-ho fails to find the USB and to meet the one who sent the text. But she finds Jung-ah’s lipstick on the floor and becomes suspicious. One day, Mi-ho coincidentally finds the USB and discovers the "Pandora’s Box" folder inside. Meanwhile, Na-young sees Tae-ho and A-ra having an affair and decides to seek revenge. Right then, someone sends an anonymous letter to the bank, exposing Mi-ho’s past.

Mi-ho finds out that the malicious social media post about her past was Jung-ah’s work and that Chang-hyun was at the apartment when Yu-jin was killed. Mi-ho thinks Jung-ah might have hired Chang-hyun to kill Yu-jin. But after Na-young tells Mi-ho that it wasn’t Jung-ah, Mi-ho is back at square one. Right then, the detectives barge in to investigate Mi-ho for attempted murder of Do-jun.

Do-jun claims that Yu-jin and Mi-ho colluded to kill him and Mi-ho is questioned by the police. Mi-ho learns from the kindergarten teacher that Yu-jin took all of Ji-yul’s drawings before she was killed, and it bothers her. While looking through anonymous posts on the moms’ club, she is shocked to see a post about a woman who thinks her child saw a strange video on her husband’s laptop.

Mi-ho is certain that the anonymous post was written by Yu-jin. She unlocks the USB with a professional’s help and finds Do-jun’s video in the "Pandora’s Box". She confronts Do-jun, but he claims that Yu-jin started to suffer from anxiety after meeting Mi-ho and committed suicide. In front of Yu-jin’s house, Mi-ho finds a package addressed to Yu-jin with a note and a key.

Mi-ho goes to see Hye-jung to find out what the key is for. But Hye-jung is hostile to Mi-ho. After pressing Hye-jung, Mi-ho finds out the shocking truth about her mother and Yu-jin's father, and feels even guiltier towards Yu-jin. Meanwhile, Do-jun finds out that Mi-ho met Hye-jung and contacts ‘...’ on his phone. Finally, Mi-ho finds out that the key is for the resident locker in the apartment.

Mi-ho finds a ripped piece of a drawing in the locker and discovers someone took the package on the day of the incident. She then concludes that Ji-ye was the one who left the package. She confronts Ji-ye, but Ji-ye only admits to looking for the USB and says she thinks Su-bin left the package. Mi-ho confronts Su-bin, but he denies it looking frightenend, which leaves Mi-ho suspicious.

Mi-ho tries to ask Jung-sik about the relationship between Su-bin and Yu-jin, but Jung-ah shows up and stops him. Later, Jung-ah tells Mi-ho about the heavy loan Ji-ye took out to buy a house and that Ji-ye has a motive. Mi-ho decides to follow Su-bin and Ji-ye and finds out that Ji-ye was at Yu-jin’s house on the night of the murder. Meanwhile, Ji-yul tells Mi-ho that she has an older sister.

While Mi-ho is headed for Marilyn to meet Ju-a, Yu-jin’s secret daughter, Jin-seop is attacked by Ju-a who thinks Mi-ho is her birth mom. Ju-a is shocked to hear Yu-jin is dead and that Do-jun is her husband. Mi-ho is certain something happened between Ju-a and Do-jun and that it led Do-jun and Yu-jin to fight. Meanwhile, when Ji-ye tries to flee after scamming people, Mi-ho catches her.

Ji-ye clams up upon Do-jun’s threat. Mi-ho and Do-jun are out to find the black USB before one another. While trying to replay what happened the night Yu-jin died, Mi-ho realizes where the USB might be and finally finds it. The USB contains audio files of Yu-jin hiring a hitman on Do-jun and him doing the same. Mi-ho is sure that Do-jun is the killer, and he kidnaps and takes her to his clinic.

Do-jun tries to kill Mi-ho. She gets away from him but gets caught and dragged up to the rooftop. Just when he’s about to throw her off the roof, Jin-seop, Jung-ah and Na-young save her. Mi-ho hears the news of her promotion and new post in an overseas branch. She reads the last part of Yu-jin’s complaint letter and lets go of her grudge. She decides to start a new life with Ji-yul and Ha-yul.