Return to Sender by Julia Alvarez is about two families, the Paquette family who live on a dairy farm in Vermont and the Cruz family who immigrated from Mexico to the United States in search of a better life. Mr. Cruz and his two brothers were hired to work on the Paquette farm (despite being undocumented) when the farm is in desperate need of workers after Mr. Paquette had a debilitating accident and the Grandfather passed away suddenly of a heart attack.
At first, Tyler Paquette is unsure about the arrival of the Mexican family on his farm. Mr. Cruz and his brothers Tio Armando and Tio Felipe were hired to work but also came with Mr. Cruz's three daughters, the three Marias: Maria Dolores (Mari), Maria Ofelia (Ofie), and Maria Lubyneida (Luby). Despite the family being kind and hard working, Tyler hears that they are "illegal", meaning they don't have proper documentation to work in the U.S. He worries his family is breaking the law by hiring the Cruzes and finds this reality conflicting. Wasn't his family always telling him how important it was to be a law abiding citizen? Along with this confusion, Tyler is also mourning the sudden loss of his grandfather, to whom he was very close. Tyler is also feeling anxious as, after his father's serious injury, the family worried that they'd lose the farm altogether. Tyler loves the farm and calls it his favorite place in the whole world. The thought of losing it is terrifying. When the Cruzes arrive to help, Mrs. Paquette calls them her angels and is so grateful that with their help they will be able to save the family farm.
The Cruz family immigrated to the U.S. together years ago and Ofie and Luby were born in North Carolina. However, the girls' mother, Señora Cruz went back to Mexico to see their abuelita before she passed away. When Senora Cruz was crossing back into the U.S. by using a "coyote" (a smuggler), something unknown happened and ever since the Cruzes have had no information about her whereabouts. Senora Cruz has been missing for over a year when the family arrives on the farm. Mari and her family miss her terribly and pray every day for her safe return.
After a few months at the Paquette farm, with the help of the Cruzes, things are steadily improving. Tyler and Mari become close friends and bond over their love of stargazing. They are able to share their hopes and fears together and Mari feels lucky to have found a special friend. The three sisters attend school and become especially close with Tyler's grandmother, who appreciates their sweet presence after the loss of her husband. They spend their afternoons after school with Grandma sharing and learning about each other's cultures.
One day, the Cruzes are shocked to discover that young Tio Felipe was arrested while out at night with Tyler's college-aged brother, Ben. Because he does not have proper documentation, Felipe is forced to remain in prison for months before being deported back to Mexico.
A few months later, the family is overjoyed to finally hear from their mother but they are heartbroken when they learn the truth. The reason they hadn't heard from Senora Cruz for so long was because she had been kidnapped by human traffickers on her journey back into the United States a year ago. The kidnappers demand a ransom and despite pooling all their money together, the Cruzes fear they do not have enough. In an effort to help, Tyler offers Senor Cruz all the money he had saved for a special school trip to Washington, D.C. With Tyler's help, the Cruzes have enough money to pay the kidnappers. Tyler and Mari journey to North Carolina with Tyler's aunt and uncle and rescue Senora Cruz. The terrible details of her ordeal slowly emerge and they find that she had been horribly abused for the past year by the smuggler.
Not long after the family is reunited, tragedy strikes again when ICE agents raid the farm! Immigration and Customs Enforcement (la migra) take Senor and Senora Cruz along with Tio Armando into custody. Because the agents believe Senora Cruz to be in league with her kidnappers and because Senor Cruz initially resisted arrest, they wind up with criminal charges as well! Fearing her parents will be in jail for months, Mari summons the courage to do what she can to set the record straight. With the help of their Spanish teacher, Senora Ramirez, and a kind lawyer named Mr. Calhoun, Mari and Tyler go to the police to plead the Cruzes' case. Mari explains about the nightmare her mother had been through when she was kidnapped. She clarifies that they are not criminals but simply a family seeking work in the U.S. She offers herself up to the police if they will let her parents go, especially her mother who has been through so much trauma already. The officer is moved by Mari's appeal and says he will do what he can.
In the end, the criminal charges are dropped but the Cruz's are still deported back to Mexico. Without their help, the Paquettes are not able to keep up with all the work needed and have to sell the farm. Even worse, Tyler is sad to lose a friend. He and Mari write to each other and encourage each other by saying that at the end of the day they are both looking at the same stars even if they are on opposite ends of the continent. Tyler writes, "We are the ones we have been waiting for ... You and me, Mari, it's up to us. We are the ones who are going to save this planet. So we've got to stay connected - through the stars above and the swallows and letters back and forth. Your friend forever... Tu amiga, para siempre."