Soundtrack #1 follows Han Sun-woo, a rookie photographer with a warm heart and few words, and Lee Eun-soo, a witty and straightforward lyricist. Having been best friends for 19 years, their lives become intertwined when Eun-soo needs to write a hit song about unrequited love and enlists Sun-woo to live with her for two weeks to work on the track together. As they spend day and night sharing a roof, unspoken feelings and hidden romantic tensions bubble to the surface. Sun-woo has secretly harbored deep feelings for Eun-soo for years, but fears that confessing will ruin their lifelong bond and cause him to lose her forever. The series explores the delicate, heart-wrenching boundary of the friend zone, blending emotional storytelling with a rich, mood-enhancing musical score.