The following version of the book was used to create this guide: Lại, Thanhhà. Butterfly Yellow. Harper, 2019.

In the year of 1981, a young Vietnamese woman named Hằng makes her way across Texas by bus in order to reunited with her long-lost brother, Linh, whom she was separated from during American evacuation efforts in the Vietnam War. At the same time, a young man from Austin named LeeRoy travels north to Amarillo in a pickup truck in pursuit of his dreams of attending a real-life rodeo. The pair crosses paths when Hằng is abandoned at a rest station by her bus driver and LeeRoy finds himself responsible for transporting her the rest of the way to Amarillo. Although Hằng remains largely reticent during their drive, she is able to communicate that she is trying to reach a particular address where she has been told her brother now resides. When they arrive at the Amarillo address, they discover that the house no longer exists, and Hằng collapses to the ground in despair.

LeeRoy is prepared to abandon Hằng and continue seeking out his yearned-for rodeo, but the pair are approached by one of the neighbors, a woman named Mrs. Brown. Mrs. Brown deceives LeeRoy into staying on the scene by confiscating his wallet, then provides Hằng with information about where Linh can be found, giving her the address of Los Cedros Ranch. Hằng beseeches LeeRoy to drive her to the ranch, but he insists on stopping by the rodeo first, and declines to allow Hằng to come inside with him. LeeRoy is promptly kicked out of the rodeo after he initiates a fight with one of the men in line, and he drives Hằng to Los Cedros in the dead of night. When they knock on the door the next morning, the woman who answers, Cora, demands that they leave her property; the situation gets out of hand when Hằng's uncle, Chú Quốc, arrives on the scene and demands that she return home with him. LeeRoy prevents this from happening by pursuing Chú Quốc in his pickup truck, but severely damages the truck in the process.

Cora's neighbor, a man named Mr. Morgan, decides to hire Hằng and LeeRoy to work on his ranch for the remainder of the summer. Cora expressly forbids Hằng from speaking with Linh, who is going by the name David, but Hằng spends much of her time finding opportunities to spy on David anyway. One night, LeeRoy hears tell of a "ghost rodeo" happening at a nearby ranch and sneaks off to attend it; his plans are foiled when both David and Hằng smuggle themselves along with him. LeeRoy suffers a serious injury at the ghost rodeo after being tricked by a pair of ranchers, and Hằng and David are made to work together in order to help LeeRoy get himself out of trouble. When Mr. Morgan learns that his employees have attended a ghost rodeo, he begins to give them increasingly punishing farm labor to perform, and Hằng starts painstakingly recording memories of her life in Việt Nam so that David can come to better understand his heritage.

Chú Quốc begins pressuring Hằng to help him retrieve legal custody of David, and Hằng finds herself at odds; she badly wants her brother back in her life, but has begun to realize that David does not want to leave Los Cedros and has grown attached to Cora and Mr. Morgan. David, Hằng, and LeeRoy begin to bond as they labor to remove a mesquite tree from the ground; Hằng draws David pictures of fruit, while LeeRoy helps Hằng diagram English sentences so that she can learn them better. LeeRoy even begins to lose interest in rodeos, attending one and discovering that he vastly prefers the company of Hằng. Chú Quốc and his daughter, Angie, come to Los Cedros in an attempt to bring both David and Hằng home with them, and Hằng earns favor with Cora and Mr. Morgan when she both refuses to go and refuses to help Chú Quốc file suit against Cora for custody of David. As the summer draws to a close and Mr. Morgan starts preparing his crop for market, the ranch is hit by a devastating hailstorm, and Hằng's scars and welts from her difficult passage to America are exposed to the other people on the ranch. LeeRoy follows her when she runs away in shame and comforts her while she processes the grief of her passage out of Việt Nam.

In preparation for the local fair where Mr. Morgan hopes to sell his crop, Cora designs Hằng an outfit that lets her cover her scars with hosiery, forging a fragile peace between them. At the fair, David is so eager to eat food and ride the amusements that he begins to open up to Hằng; as he, Hằng, and LeeRoy ride a Ferris wheel, Hằng begins to sing his favorite song from childhood, "Butterfly Yellow," and is shocked when David remembers the words. Mr. Morgan offers Hằng full-time work at the ranch as a gardener, which she gladly accepts, and Cora begins treating David and Hằng as brother and sister. LeeRoy decides to attend a nearby college so that he can study language there and one day become a language instructor. When he bids farewell to Hằng at the end of the summer season, he promises to return in the winter.