The film begins with a card that reads it’s “based on a true lie.” Billi (Awkwafina) resides in New York with her parents, Haiyan (Tzi Ma) and Jian (Diana Lin), while maintaining a close relationship with her grandmother, Nai Nai (Shuzhen Zhou), back in China. They frequently chat on the phone, and during one of these calls, Nai Nai fusses over her, as most grandmas do. However, Nai Nai keeps a significant secret from Billi—she’s in the hospital for tests, along with her sister, Little Nai Nai (Hong Lu). After her scan, Nai Nai learns from her sister that there’s nothing to worry about, just “some benign shadows.”
Billi is navigating a rough patch in her life. Later that day, while speaking with Nai Nai, she receives the disappointing news that her application for a Guggenheim fellowship has been rejected, but she chooses to keep this from Nai Nai and everyone else. Billi visits her parents, where her mother scolds her about her financial struggles and lack of direction in life. Jian informs her that they are heading to China the following day for her cousin Hao Hao's wedding to Aiko, a Japanese girl who doesn’t speak Chinese, and that Billi is not invited.
When Billi questions the haste of the wedding, she is shushed since her father is resting. Intrigued, Billi confronts her father and senses something is wrong. When pressed, he reluctantly reveals the heartbreaking fact that Nai Nai has been diagnosed with stage four lung cancer, and the wedding is merely an excuse for the family to visit her. Despite her desire to go, her parents are adamant that she stays behind, believing that revealing Nai Nai’s condition will only cast a shadow over her remaining days. They fear Billi’s expressive nature would betray the truth.
To everyone's surprise, Billi travels to China despite her family’s disapproval, bringing joy to Nai Nai. Jian again questions where Billi acquired the funds for her flight, but Billi simply demonstrates her ability to hide her feelings. As the family embarks on wedding preparations, they engage in conversations highlighting the stark contrasts between American and Chinese cultures. Amid the tensions, Nai Nai intervenes, asserting that they should never criticize China.
Nai Nai introduces Billi to a breathing exercise, contextualizing it with “you can do it alone when you go back to America.” However, Billi struggles with the family’s decision to conceal Nai Nai’s diagnosis from her, continuously bringing it up, only to be met with silence. After a night of heavy drinking amongst her family, Nai Nai suffers a coughing fit and returns to the hospital. Billi approaches the doctor, who advises that they continue to keep Nai Nai in the dark about her illness. The doctor falsely attributes her cough to a past pneumonia, assuring them it’s merely a secondary infection and recommends more tests.
Billi finds her father and Uncle Haibin (Yongbo Jiang) smoking at their hotel. She expresses her frustration about lying to Nai Nai, to which Haibin explains the cultural differences between America and China: in America, individuals own their lives, while in China, people are part of a collective family, making it their duty to bear the emotional weight of a loved one's sickness.
Determined to be there for Nai Nai, Billi tells her mother she wants to stay in China to help, but Jian brushes her off, criticizing her for her lack of domestic skills. Billi shares her unresolved feelings about missing her grandfather’s funeral, but this only prompts further dismissal from her mother.
As the wedding approaches, Nai Nai requests that Billi be gracious and kind to the guests, asking her to smile and even agree to sing if called upon. On the wedding day, after a brief dispute about the timing of her X-ray results, Nai Nai reminds Billi to keep smiling and engaging with the guests. In the midst of the celebrations, Haibin gives an emotional speech, claiming he is crying from joy. Billi, encouraged by her father, delivers a speech of her own, with impromptu karaoke to “Killing Me Softly” that lights up the room.
When the time arrives for the family picture, Nai Nai wonders why the groom is in tears, and again they tell her it’s from happiness. Little Nai Nai asks to take a photo with her camera, to which Nai Nai says she entrusted it to Mrs. Gao (their maid) to hold. As the photo is taken, Billi rushes to the hospital to speak with Mrs. Gao, only to discover she cannot read and does not know the test results. Coordinating with Little Nai Nai, they manage to alter the results to reflect “benign shadows” for Nai Nai’s sake.
When Nai Nai finally receives the modified test results, she surprises everyone by declaring, “You worried for nothing! I told you I was fine!” Before Billi departs, Nai Nai gifts her an envelope filled with money, insisting Billi deserves it regardless of not being wed. Billi reluctantly reveals her fellowship rejection, and Nai Nai imparts wisdom, saying that life isn’t about what you do but how you do it.
As Billi and her parents leave, Nai Nai embraces Billi tightly, assuring her that they will meet again. Back in New York, Billi grapples with uncertainty about her future. While walking the streets of the city, she suddenly stops and, reminiscent of Nai Nai, loudly grunts—a symbol of her connection to her beloved grandmother. The film closes with a touching video montage of the real Nai Nai, still vibrant and thriving six years post-diagnosis.
