Original: 빨간풍선
Two women — best friends since high school — realize the limits of their friendship as secrets, jealousy and desire come between them.
Eun-kang is Ba-da's lifelong friend of 20 years. She helps Ba-da, a jewellery designer, by doing odd jobs for her. When Eun-kang's boyfriend, Tae-ki, whom she had been taking care of for four years, passes the civil servant exam, Ba-da and her husband, Cha-won, take Eun-kang and Tae-ki to a luxury vacation house to celebrate. However, Eun-kang's hidden desires slowly begin to show.
Eun-kang goes swimming in the pool after becoming frustrated from an argument with Tae-ki, and Cha-won watches her from a distance. Meanwhile, Ba-da and Cha-won go through a rough patch due to Cha-won's parents. Later, Eun-kang dresses up to see Cha-won when Tae-ki pushes her away.
Eun-kang and Cha-won ignore Ba-da's calls and spend time together. A falling apart between Cha-won and Ba-da begins due to Cha-won's mother, which Ba-da can't take any longer. Meanwhile, Eun-san and Nam-chul have dinner together after working overtime, and later, Eun-san witnesses Tae-ki cheating on Eun-kang.
Eun-kang and Tae-ki's relationship becomes worse after Tae-ki meets her parents. Although Eun-kang desperately tries to hold on to Tae-ki, it's no use. Tae-ki even goes to see Ba-da and Cha-won and tells them about the breakup. Meanwhile, Cha-won's patience gets pushed to its limits as the clash between Ba-da and his mother deepens.
Eun-kang's family goes after Tae-ki for revenge after learning their wedding got canceled. Nam-chul lets out all the sorrow and pain he has been holding at Mul-sang, and Eun-san worries more about Nam-chul as she learns more about his inner feelings and his life with his in-laws. Meanwhile, Eun-kang, who has broken up with Tae-ki, makes a big decision.
Nam-chul's sudden demand causes a stir among the family members. Ba-da finds out that Gong-ju and Eun-kang met separately, so she asks Eun-kang what she said about her to Gong-ju. Eun-kang decides to show up at Tae-ki's wedding in a white dress. Meanwhile, Eun-san feels strongly attracted to Nam-chul who keeps making her worry.
Eun-kang and Cha-won spend time together at the beach in Sokcho. When Cha-won returns home from Seocho, he gets into a big fight with Ba-da about his mother-in-law. Meanwhile, Nam-chul and Eun-san become more affectionate and develop feelings for each other after sharing a kiss. However, one day, Mul-sang visits the office and notices a ginkgo tree leaf on Eun-san's desk that he had found in his camper.
Cha-won becomes confused when he learns about Ba-da's secret, and to make matters worse, the police visit his house. Later, Ba-da goes on a short getaway with her mom and runs into an old friend. However, a strange atmosphere flows between them. Meanwhile, Mul-sang makes a surprise visit to the company while Nam-chul and Eun-san are together, and in the end, has a deep talk with Nam-chul.
While Ba-da spends time with her mother on a short vacation, Eun-kang and Cha-won have a wonderful time at the amusement park. However, that night, Cha-won learns Ba-da met Ki-han during her trip. Meanwhile, Nam-chul goes to see Eun-san, and Mul-sang follows him secretly.
Cha-won asks Eun-kang about Ba-da and Ki-han's relationship after seeing them together. Meanwhile, Jeon-hee records Ba-da's shocking remarks and calls Ba-da and Cha-won together for them to listen to it. Mul-sang, who has now confirmed Nam-chul's affair, sits with Nam-chul for a talk, and Geum-ah starts to snoop through Nam-chul's things. However, as Nam-chul falls deeper into his unfaithfulness, Eun-san's greed grows.
In the end, Eun-kang and Cha-won cross the line. The next day, Ba-da finds Eun-kang's handkerchief in Cha-won's jacket pocket, and Cha-won avoids sleeping with Ba-da. Meanwhile, Mul-sang keeps pushing Nam-chul to end things with Eun-san quickly, and ultimately, Nam-chul gets forced to call her in the middle of the night. The next day, Eun-san shows up in front of Mul-sang's house.
After learning about Nam Chul's affair from Eun San directly, Geum Ah charges into their studio apartment and unleashes her rage. Eun San can't accept breaking up with Nam Chul and tells him to divorce his wife. Meanwhile, Ba Da gets completely intoxicated at the party and returns home on Ki Han's back. Cha Won, who has lost all his patience, explodes at Ba Da with everything he has held back.
Everyone learns about Nam Chul's affair, and Mul Sang's house turns into a battlefield. Eun Kang forcefully tells Nam Chul to end it with Eun San. Meanwhile, Kwon Tae Ki runs into Ba Da and shows her a shocking video of Eun Kang. Later, Cha Won and Eun Kang can't forget about each other and kiss. However, someone is watching.
Ba Da's business gets hit with an unexpected wave and sways out of control. Ba Da becomes on edge and lashes out at Cha Won with harsh words. Meanwhile, Jeon Hee wakes up after fainting from seeing Cha Won and Eun Kang kissing and tells Ba Da everything she saw. Later, Eun San runs into Nam Chul's children while still unable to accept the breakup.
Ba Da slaps Eun Kang and Cha Won at a cafe and rages at them after finding out about their relationship. Later, Ba Da hands Cha Won divorce papers, and Mul Sang, who knows about the ordeal, flips his household upside down in anger. Meanwhile, Nam Chul and Eun San bump into each other in front of the studio apartment on a snowy day and say their goodbye one last time.
Eun Kang becomes annoyed and agitated as she watches Cha Won look desperately for Ba Da. Mul Sang takes matters into his own hands to keep Eun Kang away from Cha Won. Meanwhile, Eun San's family goes crazy after learning Eun San is Nam Chul's lover, and Dae Bong punches Nam Chul in the face.
Cha Won hesitates as Eun Kang holds on to him, saying they shouldn't end things. Jeon Hee shows up at Mul Sang's house and causes a commotion in her anger. Dae Bong has a hard time and blames himself for everything after learning the facts. Meanwhile, Ji Ok exposes Geum Ah's affair with Dae Geun to Nam Chul.
Eun Kang receives good news. As she celebrates with Cha Won, Eun Kang runs into Ba Da. Jeon Hee visits Cha Won and tries to persuade him to get back together with Ba Da. Meanwhile, Nam Chul tells Geum Ah not to take it too far but explodes in anger, and Ba Da hears the shocking truth after meeting the culprit who stole her jewellery designs.
Eun San plans to leave for volunteer work overseas, while Nam Chul and Geum Ah opt for divorce. Mul Sang finds out Geum Ah has been meeting with Dae Geun again and orders Dae Bong to teach him a lesson after calling Dae Geun out to a secluded spot. Meanwhile, Ba Da sues Eun Kang and plans to seduce Cha Won again for her revenge.
Nam Chul and Eun San reunite and shed tears of joy together. Mul Sang household is on the verge of losing their fortune in a scam. Cha Won meets with Tae Ki and asks about Eun Kang's past, and Ba Da takes Cha Won's family to Eun Kang's house. Meanwhile, as Eun Kang prepares for her first day as a school teacher, she makes her final decision.
