The opening episode of Dear X unfurls with a poignant narration from Baek Ah-jin, who relives a childhood memory with poignant detail. Little Ah-jin's mother, an alcoholic, inflicts violence upon her. Her father, indifferent until a fateful day, drags her mother out into the street and shoves her down a staircase, muttering a promise of a wealthy, new stepmother if Ah-jin would lie and claim she fell while drunk. As her mother pleads for help, pleading for Ah-jin to call 911, the young girl walks away unmoved.
The scene shifts to the present, where a glamorous adult Ah-jin emerges from a car, stepping onto a red carpet like a star. The on-screen text reveals her as a top actress with antisocial disorder, manipulating those around her for her own gain. She is partially responsible for two deaths and poised to carry out a third in the near future.
The narrative then shifts to 2016, where Ah-jin is in high school, and her classmates are preparing for a birthday party for their teacher, Ms. Kwak. The celebration is abruptly interrupted when it is discovered that someone has posted photos of Ms. Kwak having an affair on the school message board. As Ms. Kwak shyly leaves the school, Ah-jin asks if she enjoyed her birthday gift, hinting at revenge for when Ms. Kwak called her an orphan.
The focus then turns to Sim Sung-hee, Ah-jin's X. Sung-hee is upset because Ah-jin has received the first rank and pushed her to second place.
She had recently witnessed Ah-jin conversing with the art teacher, and when she inquired about it, Ah-jin's demeanor turned chilly, cautioning Sung-hee about the consequences of prying noses. Meanwhile, Joon-seo, with a heart captivated by Ah-jin, had even relinquished his spot on the debate team to make room for her. As a result, Ah-jin found herself working alongside Sung-hee and two of her friends. Sung-hee, dissatisfied with Ah-jin's interference, confided in her mother, who promised to address the matter.
Ah-jin lived a solitary life in a cozy apartment, where Joon-seo met her one day and discovered a love letter intended for Ah-jin in his bag. The next morning, Sung-hee unleashed a scathing lie across the school, announcing to all that Ah-jin was an orphan. Furthermore, her mother had lodged a complaint with the school, leading to Ah-jin's removal from the debate team. In retaliation, Ah-jin presented Sung-hee with a letter bearing Joon-seo's response to her confession—a string of insults penned in Ah-jin's handwriting.
Later on, the much-anticipated debate competition was abruptly canceled due to a leak revealing that the judges had been bribed. Sung-hee, assuming it was Ah-jin behind the leak, took revenge by dousing her with a bucket of water in the bathroom.
In parallel, it emerged that Ah-jin was running a loan shark operation with another student named Kim Jae-o. The duo had forged their partnership after Ah-jin caught Jae-o red-handed stealing a student's AirPods. Currently, Jae-o was at a student's mother's restaurant, demanding payment for the debt her son owed him.
As Ah-jin emerged from the rain, drenched to the bone, Joon-seo swiftly led her to the nurse's office. There, she assumed a demeanor of timidity towards Sung-hee, imploring Joon-seo for assistance in navigating the predicament. Meanwhile, Jae-o's negligence with his book, brimming with details on the loans, caught the eye of a teacher, and in the ensuing commotion, Sung-hee inadvertently snatched his phone.
Later, Jae-o reassured Ah-jin that he had not betrayed her, and she returned the gesture with a hug. However, Sung-hee witnessed this exchange and realized that Ah-jin was the mastermind behind their plan. The next day, Sung-hee cunningly spilled juice on Ah-jin's lunch, following her to assert her authority as Kim Jae-o's boss. Joon-seo intervened, drawing Sung-hee aside and feigning interest in her, declaring a desire to date her. Yet, he cautioned her against confronting Ah-jin, citing her as a cherished friend.
Elsewhere, Kim Jae-o received a call from Ah-jin, who played the role of the pitiful damsel in distress. Jae-o inquired what he could do to help her, and they concocted a scheme. In school, Ah-jin sneaked a peek at Sung-hee's locker combination while Joon-seo persuaded Sung-hee to skip their Physical Education class in favor of a hangout session. Sung-hee agreed and soon lied to the teacher about visiting the nurse's office.
Upon reaching their classroom, she found a note directing her to the art room, which she assumed was a ruse by Joon-seo. She proceeded there, only to find Ah-jin holding hands with the art teacher. Meanwhile, Jae-o sneaked into Sung-hee's locker and placed an envelope stuffed with cash—collected as a wedding gift for the homeroom teacher—inside it.
When everyone returned to the classroom, the students realized that the envelope with the cash was missing. They suspected Sung-hee and eventually discovered the cash and Jae-o's phone in her locker. Desperate now, Sung-hee blurted out that Ah-jin was dating the art teacher. This incited an angry reaction from the home room teacher, whose fiancé was none other than the art teacher himself! Ah-jin effortlessly dismissed their conversation as a discussion about a counseling center called True Love!
Jae-o then posted a video of the entire incident online, which even landed her parents in trouble. Later, Sung-hee persisted in her belief that she could still prevail with her wealthy parents as her backing. But Ah-jin revealed a shocking truth: the teacher from their initial encounter was having an affair with Sung-hee's father. Once that news became public, Sung-hee's father would likely lose his job, and she would no longer wield as much influence as before.
