The narrative begins with lyricist Lee Eun-soo facing rejection from composer Kang Woo-il, who points out that her lyrics about unrequited love sound fake and emotionless. Desperate to write a hit song, Eun-soo asks her best friend of 19 years, photographer Han Seon-woo, to live with her for two weeks and help her understand one-sided longing. Seon-woo has secretly loved Eun-soo for eight years, a fact he hid because Eun-soo once declared that friends should never date if they want to stay together forever.
During the cohabitation, intimate domestic moments begin to erode their platonic boundaries. The major turning point occurs in Episode 2, when Eun-soo browses Seon-woo’s computer to look at photography files and discovers a hidden archive of candid photos of herself dating back several years. Realizing he loves her, Eun-soo is hit with deep confusion and guilt. Rather than confessing immediately, Seon-woo decides to leave for an overseas photography assignment to give them both space.
In Episode 4, with Seon-woo away for months, Eun-soo finally experiences the deep, hollow ache of missing someone without knowing where their relationship stands. Having finally understood the pain of unrequited love, her lyrics achieve immense success. When Seon-woo makes a surprise visit back to Korea, Eun-soo writes a new verse in her lyric notebook confessing her love and presents it to him. Seon-woo takes the pen, writes his own mutual confession on the paper, and the series concludes with their joyful first romantic embrace, transforming their two-decade bond into romantic love.