It’s the summer of 2009 and Cheong Ah is a senior in high school who is the second of three daughters. Her parents run a small chicken restaurant while her older sister is a news anchor. Cheong Ah doesn’t have any hopes or dreams, and no one knows why, except for a friend she has only talked to via text messages. She skips school and gets on a train to go meet him where she runs into a handsome soldier.

The friend Cheong Ah plans to meet at the train station is Joon Kyum, a boy from a well-off family. The two teens plan to commit suicide together. They enjoy their last meal together and return to the small room they have rented, where Cheong Ah falls asleep. When she wakes up in the middle of the night, Joon Kyum is gone and only a letter remains. Cheong Ah calls her mom for help, who tells her to make everything look like an accident.

Cheong Ah must make a witness statement at the police station about Joon Kyum’s death, but she is still in shock. Her mom takes the lead, making things up as she goes. Judge Hong comes to the police station after identifying her son’s body. She is not only upset about her son’s death, but also suspicious of Cheong Ah’s story.

At home, Cheong Ah confesses why she wanted to end her life. Meanwhile, Seol Ah is searching for a fashion brand that will sponsor her wardrobe, but to no avail. However, Jin Woo overhears Seol Ah talking to the store employee and decides to sponsor her wardrobe personally. Finally, Cheong Ah must sneak out Joon Kyum’s letter from police evidence.

While Cheong Ah is on her way out of the police station with Joon Kyum’s letter, Judge Hong bursts in wearing her pajamas. Judge Hong tries to search Cheong Ah and when that doesn’t work, she requests an autopsy on Joon Kyum’s body. Meanwhile, Seol Ah pieces together the story and figures out what is going on. Later on, the police come to search Cheong Ah’s room.

Seol Ah sees her mom trying to hide Cheong Ah and Joon Kyum’s phones. She tells her mom that an inquiry at the phone service provider will reveal the call and text histories anyway. Meanwhile, Jin Woo brings in a reporter to spin the story of his cousin’s death in a favorable way to the family business. As the press begins to hound Judge Hong, she reconsiders the investigation.

The day of Jun Gyeom’s funeral arrives. Judge Hong calls Cheong Ah’s mom, Young Ae, and finds out that Cheong Ah went to school. However, Cheong Ah shows up at the funeral on her own. Chairwoman Hong, Jun Gyeom's aunt, takes her anger out on Cheong Ah. Meanwhile, Seol Ah goes on a blind date, but the man who shows up is not the person she expected to see.

Seol Ah’s blind date turns out to be better than she expected. However, when she comes home, she figures out that her date is related to Jun Gyeom. Seol Ah blames Cheong Ah for ruining her chances of marrying a rich man. Meanwhile, Judge Hong invites Cheong Ah to her house and asks to know the truth about Jun Gyeom’s death. Finally, a year goes by and Jun Hwi receives a strange letter.

Nine years after Jun Gyeom’s death, Cheong Ah studies to become a police officer. She takes the civil service exam for the seventh year running, but things don’t seem to go very well. Still, Cheong Ah tries hard, remembering Jun Gyeom’s last words to her. Meanwhile, Seol Ah lives a harrowing life as Vice Chairman Do Jin U’s wife, because she still doesn’t have a baby.

Cheong Ah’s friend Baek Rim asks her to fill in for him on a part-time job. She goes to the park, dressed as Mr. Lightning, to hand out flyers. Meanwhile, Jun Hwi, who now works at Inter Market’s Sports Marketing Department, is at the same park for a promotional event. Jun Hwi mistakes Cheong Ah as someone his team hired for the event. Finally, Seol Ah gets shocking news while visiting the hospital.

Jun Hwi visits Cheong Ah’s studio apartment in search of Rim, who he thinks is Mr. Lightning. Suddenly, Cheong Ah gets a call from the hospital. Rim is the victim of the car accident that Jin U caused, but he isn’t badly hurt. Jin U, on the other hand, is still unconscious after surgery. However, Seol Ah discovers that Jin U is having an affair. Seol Ah tries to keep Jin U’s affair a secret from Chairwoman Hong.

Jun Hwi visits Cheong Ah’s studio apartment in search of Rim, who he thinks is Mr. Lightning. Suddenly, Cheong Ah gets a call from the hospital. Rim is the victim of the car accident that Jin U caused, but he isn’t badly hurt. Jin U, on the other hand, is still unconscious after surgery. However, Seol Ah discovers that Jin U is having an affair. Seol Ah tries to keep Jin U’s affair a secret from Chairwoman Hong.

Cheong Ah shows up at Rim’s hospital room in the Mr. Lightning costume to prove her innocence to Jun Hwi. They go to the patrol division to let the police know what happened. After Cheong Ah’s name is finally cleared, Jun Hwi asks the Head of Patrol about the detective who handled Jun Gyeom’s death several years ago. Meanwhile, Seol Ah is in shock after hearing Jin U’s condition.

Cheong Ah and Jun Hwi return to the hospital together. Jun Hwi tells her to come by his office the next day to discuss the payment she owes for damaging the camera. Cheong Ah ends up telling Jun Hwi about Jin U’s car accident. Meanwhile, Chairwoman Hong is busy protecting Jin U’s assets from Seol Ah, who takes desperate measures to keep her place as Jin U’s wife.

Cheong Ah and Seol Ah run into each other in front of the ICU and they realize that they are both involved in the same car accident. Seol Ah tells Cheong Ah to keep their reunion a secret from their mom. Tae Rang sees Seol Ah waiting in front of the ICU and forces her to go eat with him. As they fall asleep in the hospital chapel, Jin U’s condition worsens.

Yeong Woong sulks over his wife’s supposed mistreatment of him and he sneaks out with Yeon Ah’s gold medal to replace with a fake. Jun Hwi submits a repayment contract regarding the camera that Cheong Ah damaged, but his boss is not happy about it. Jun Hwi calls Cheong Ah into the office to sign the contract. Meanwhile, Young Ae visits an OBGYN at the hospital where Jin U is being treated.

Cheong Ah runs into Chairwoman Hong in front of the elevators at Inter Market. Before Chairwoman Hong remembers who she is, Cheong Ah makes a run for it. Meanwhile, Young Ae discovers that she needs surgery. As she sits near the hospital, thinking about what the doctor told her, she sees Seol Ah for the first time in nine years. They have a brief, tearful reunion.

Seol Ah discovers Jin U’s generosity toward Hae Rang and her family and becomes even more enraged. However, Chairwoman Hong continues digging for evidence of Jin U’s affair to use as an excuse to get Seol Ah divorced from her son. Meanwhile, Young Ae discusses the accident with Cheong Ah and says that they must take Seol Ah’s side. Jun Hwi’s search for the girl from Jun Gyeom’s death continues.

Young Ae and Judge Hong meet in the lobby of the hospital. When Seol Ah shows up to meet her aunt-in-law, she’s surprised to see her mom there but pretends not to know her. Young Ae leaves the hospital in a hurry, but her legs give in from the shock of the encounter. Tae Rang sees her and helps her get home. Meanwhile, Cheong Ah confides in Jun Hwi regarding the accident.

Tae Rang brings home the food that Young Ae made for Seol Ah and delivers it the next day. Seol Ah thinks Tae Rang must have some other motive, clueless that the food is actually from her mom. Meanwhile, Chairwoman Hong continues to threaten Seol Ah with divorce, using the picture of Hae Rang and Jin U as proof of Jin U’s intentions. Finally, Judge Hong visits Cheong Ah at her studio apartment.

Cheong Ah and Jun Hwi meet at the accident site by chance. Cheong Ah tags along with Jun Hwi, whether he likes it or not. As they find evidence from the accident together, both Cheong Ah and Jun Hwi discover more about each other. Meanwhile, Seol Ah convinces Chairwoman Hong that they should be on the same side.

Tae Rang receives a phone call from Young Ae, asking to deliver another meal to Seol Ah. At the office, Chairwoman Hong bursts into Jun Hwi’s office and warns him to stay away from Cheong Ah. Meanwhile, Cheong Ah visits her mom, who gives her an envelope of money. While leaving her parents’ house, she runs into Mr. Cha, Chairwoman Hong’s secretary.

Seol Ah eats her mom’s food that Tae Rang delivers to her. When Chairwoman Hong sees Seol Ah eating, she suppresses her anger and begins her nice mother-in-law act, just as her lawyer suggested. Meanwhile, Yeong Woong comes home from the local welfare center empty-handed and tries to blame it on Young Ae, as usual. However, this time, Young Ae takes a stand.

Cheong Ah receives a phone call from Judge Hong, who asks Cheong Ah to join her for dinner. Cheong Ah hesitates, wondering whether she should meet Judge Hong or run and hide as Seol Ah told her to. Meanwhile, Jun Hwi gets called up to Chairwoman Hong’s office. He becomes worried that the chairwoman could have threatened Cheong Ah as well and goes to see her at the studio.

Hae Rang wakes up from her coma and Seol Ah finally finds out that Tae Rang and Hae Rang are siblings, not husband and wife. Seol Ah feels betrayed that she opened up to Tae Rang. Meanwhile, Inspector Mun decides to help Jun Hwi investigate the car accident. Finally, Rim and Jun Hwi meet outside Hae Rang’s hospital room.