This description contains spoilers. Set in Southern Vietnam in 1997, The Girl from Yesterday is a story about a student named Thu. While Thu is a little boy, he prides himself on being smart and on his ability to playfully bully the girl next door, Tieu Ly. However, once Thu grows up, he becomes miserable and inconsolable about his classmate Viet An, for whom he has feelings. The film began ten years prior to the present. During Thu's younger years, he often spent time with Tieu Ly. While they mostly get along well, Thu often bullies her. Eventually, Tieu Ly and her family move to a new home, which causes a heartbroken Thu to keep a secret notebook serving as a keepsake for his and Tieu Ly's relationship. Thu borrows a book from his classmate Viet An and intentionally writes a song lyric in it. Upon discovering it, Viet An tells Thu to get her a new book. Thu borrows another book and stays up until three in the morning to finish it, as Viet An wants him to give it back as soon as possible. Knowing that Viet An likes candy, Thu decides to buy candies for her every day. However, Viet An has two close friends, Chieu Minh and Hong Hoa, so Thu has to buy candies for all three of them, causing him to run out of money. Finding it hard to win Viet An's heart, Thu asks for advice from his best friend Hai, yet still fails in his pursuit. As Thu continues to fall for Viet An, he begins to miss Tieu Ly. Viet An becomes sick and unable to attend school, so Thu visits her and voluntarily takes notes on all the lessons she missed. As he focuses on writing lessons for Viet An, he ends up forgetting about his own lessons and is scolded by his teacher. At the same time, the teacher discovers a love poem Thu wrote about Viet An. Viet An scolds Thu about the poem but asks him to go to the movies with her. Joyfully thinking that only the two of them will go, it transpires that Viet An has also told Hong Hoa to come along. The film is not only about Thu and Viet An, but also about the love between Chieu Minh and her exercise teacher, Luc. They love each other, but Chieu Minh has been diagnosed with an unpredictable heart defect. Knowing Chieu Minh's condition, Luc brings her to the city for proper treatment. Thu has a misunderstanding with Luc, the exercise teacher, which leads to a fight. The fight causes Thu to be suspended. Hai and Viet An come to Thu's house to teach him the lessons that he has missed due to his suspension, allowing him to keep up with schoolwork. Over time, Thu and Viet An become a couple, as do Thu’s best friend, Hai, and Viet An’s close friend, Hong Hoa. Thu takes Viet An to his house and shows her his secret notebook. Thu misunderstands that Viet An uses papers from his notebook to make paper boats, so he angrily tells Viet An to leave his house. He later realises they're just blank papers. Thu chases Viet An to get the notebook back. As they're on the bridge, Viet An reveals that she is, in fact, Tieu Ly, which surprises Thu. Thu is happy to meet his soul-mate childhood friend again.
