The film revolves around iconic musician Trịnh Công Sơn and the muses that have come and gone in his life, from his passionate youth in the 1960s to his later middle-aged 1990s.

Trịnh Công Sơn's youth, as a young romantic on his way to become one of the greatest songwriters about love and sorrow. Originally set to be a paired release alongside "Em and Trinh."

"Ngày Xưa Có Một Chuyện Tình" revolves around the intertwined friendship and love story of two boys and a girl from childhood to adulthood, as they face the challenges of fate. Spanning four stages from 1987 to 2000, the three friends—Vinh, Miền, and Phúc—grow up together, experiencing the innocence of love, life's struggles, and personal growth along the way.

　　Under the drizzling rain of the bustling Saigon city, a dreamy musician and a hard-headed realistic girl stumble across each other and fall in love. In the face of economic, social, familial, and romantic adversity, they are yet to answer their own questions on how to have a happy life.