In the summer of 2014, the story of "Still Shining" begins as Yeon Tae-seo departs a study center in bustling Seoul. His aunt drops off his younger brother, Hui-seo, at the train station, and the two brothers embark on a journey to their grandparents' hometown, nestled outside the city's limits. Given Hui-seo's fractured leg, Tae-seo carries him through their trip, a silent testament to their unbreakable bond.
Upon arrival, Hui-seo pleads with their grandfather, Chang-sik, to allow Tae-seo to attend the local school. At school, Tae-seo narrates their recent relocation to Seoul and the subsequent bullying that Hui-seo endured, leading them to seek refuge with their grandparents until graduation. Impressed by Tae-seo's academic achievements, the teacher encourages him to prepare for the PSATs during the summer break. He tasks Tae-seo with collecting uniforms from the donation box near the library and mentions Mo Eun-a, a fellow student who has been studying independently before the exams.
Inside the library, Tae-seo catches a glimpse of Eun-a but chooses to remain silent. After collecting the uniforms, he returns home to find out that his study table has been delayed. The next day, Tae-seo decides to study at the library to escape the distractions at home. There, he encounters Eun-a again, her loud and chatty nature initially irritating him, though he keeps his thoughts to himself.
During lunch break, Eun-a invites him to join her in the break room for ramen. She talks incessantly while Tae-seo listens in silence. Later that evening, Eun-a inadvertently locks the library before leaving. Tae-seo tells her that he plans to study at home from then on. As Eun-a cycles back and Tae-seo walks toward the bus station, he notices her catching fireflies and is transported back in time to a childhood visit with his parents. During that visit, Hui-seo had joined Eun-a in catching fireflies while Tae-seo walked home with their parents.
The next morning, Tae-seo waits outside the library for Eun-a to unlock the door. While chatting, he suggests hiding the key somewhere so he can enter if she arrives late, but Eun-a refuses, saying she feels a sense of responsibility for the library. Despite the intense summer heat, they continue studying there together, forging a unique bond amidst the hustle and bustle of their daily routine.
As they departed that evening, Tae-seo mentioned that he had brought a bicycle so they could ride home together. However, Eun-a revealed that she had taken the bus that day. Soon after, they ended up racing their bicycles. Tae-seo initially won but slowed down when he saw Eun-a's disappointment, allowing her to finish first. Over the next few weeks, the two spent more time together and gradually developed feelings for one another.
One evening, Eun-a returned home to find her father, Mo Seon-gyu, hosting international guests with his partner, Park So-hyeon. Later that night, Seon-gyu cooked dinner for her. Eun-a scolded him for drinking too much and asked about his girlfriend, but he claimed So-hyeon was busy entertaining the guests. Eun-a accepted the explanation and finished her meal.
The following morning, heavy rain woke Eun-a up. After giving her father a hangover drink, she suddenly realized that Tae-seo must be waiting at the library. She cycled to the bus stop to hand him the key, leaving one slipper behind in her rush. Tae-seo noticed this but accepted the key and promised to wait for her later.
At the library, he placed drinks in the fridge for them to share. When Eun-a finally arrived, she admitted that she had been worried about her father earlier that morning and said she felt calmer whenever she looked at Tae-seo. To reassure her, he sat across from her while they studied.
Later, Eun-a texted So-hyeon but became upset when she learned that So-hyeon would not be visiting her father. Concerned, she rushed home, leaving Tae-seo alone at the library. Unable to concentrate, Tae-seo decided to visit her house. On the way, his grandmother Hwa-soon spotted him and invited him to lunch with her friend. That night, Tae-seo wondered whether Eun-a would return to the library the next day. When he unlocked the door the following morning, he heard her footsteps and visibly relaxed.
While studying together, Eun-a discovered several name tags belonging to girls from Tae-seo's former class in Seoul. Embarrassed, Tae-seo insisted he had no idea they had slipped them into his bag as farewell gifts. However, Eun-a teased him and refused to believe his denial. During lunch, Tae-seo asked why she had left so suddenly the day before. Eun-a revealed that her father suffers from anxiety and depression and once attempted to end his life. Consequently, she worries whenever he is left alone. She added that his condition had been improving since he started seeing a therapist.
Later, they talked about their future plans. Tae-seo expressed his hope to attend college far from home so that he could support his grandparents and brother without feeling overwhelmed by being nearby. Eun-a realized that she would rather stay close to her father than move to Seoul for university, as she wanted to be there to help him if something happened.
Reflecting on the month they spent studying together, Tae-seo explained that he must work hard to become the family's breadwinner after their parents' death. Eun-a mentioned her interest in drawing, and Tae-seo reassured her that she had a promising future in art. In a moment of nervous honesty, Eun-a accidentally admitted that she liked him, leaving both of them awkward.
That night, they cycled home and prepared for the first day of school. The next morning, the teacher asked Tae-seo to continue studying in the library for an elective course that the school didn't offer. As the months passed, both Tae-seo and Eun-a focused on their studies.
One day, Tae-seo took Hui-seo to the hospital to have his cast removed. Hui-seo worried that he might never walk properly again, but he kept his fears from their grandfather. Around the same time, Tae-seo began experiencing occasional hearing problems but didn't pay much attention to them.
A month later, Eun-a unexpectedly ran into Tae-seo while they were both traveling to Seoul. Excited to see him, she suggested they spend time together, but he refused, claiming he had urgent business in the city. Instead of spending time with Eun-a, Tae-seo attended an admission seminar at Hanguk University.
Meanwhile, Eun-a visited So-hyeon at her apartment and was shocked to find her with another man. Assuming she was cheating on her father, Eun-a confronted her. So-hyeon explained that she was not officially dating Seon-gyu and that it was Eun-a's father who refused to define their relationship. Upset and with nothing left to do in the city, Eun-a returned to the train station.
While in Seoul, Tae-seo met a friend who offered him a tutoring job at his father's study center starting the following February, before university began. Later, Tae-seo visited a doctor about his hearing issues and was told they were likely stress-induced and occurred only occasionally.
Tae-seo talked to the doctor about his parents' car accident when they were out shopping with Hui-seo. He anxiously confessed to experiencing trouble with his hearing since the accident, which also led to Hui-seo's leg injury.
Feeling lonely, Tae-seo called Eun-a. She was upset with him for being distant and rude to her as she broke into tears. Realizing his mistake, Tae-seo rushed to the train station where she was waiting. He found her standing by the Han River, and the two took the train home together.
During the ride, they finally talked openly about their feelings and how being together helped them feel calm. Eun-a said they should stay together forever, and Tae-seo agreed. The episode ended with Tae-seo holding Eun-a's hand as they officially began dating.
