Battle for Happiness

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.