Summer of the Mariposas is framed as a Mexican American retelling of Homer’s Odyssey, as the five Garza sisters embark on a magical quest to heal old wounds and rediscover what it means to be a family. In the prologue, 15-year-old Odilia—the eldest Garza sister—describes the summer that her hometown of Eagle Pass, Texas experiences a massive influx of butterflies (mariposas). That summer also marks the one-year anniversary of her Papá’s abandonment of the family, leaving Mamá to support Odilia and her four sisters alone. The Garza girls spend most of that summer swimming in a quiet alcove of the Rio Grande, until one day a dead man floats into their swimming hole. Despite Odilia’s insistence that they need to alert the authorities, 14-year-old Juanita believes the more honorable course of action is to return the dead man—whose name is Gabriel Pérdido—to his family in El Sacrificio, Mexico. She contends that the man’s must be worried about him, thinking of Papá’s own unexplained disappearance. The twins, Velia and Delia, are quick to jump on board, as is the youngest sister, Pita, when she learns that Papá’s mother—Abuelita Remedios—lives near El Sacrificio, meaning the girls can stop for a long overdue visit. Though Odilia tries to stop her sisters, she ultimately goes along with their harebrained plan because of their sisterly code to stay together forever no matter what.
While her sisters prepare the dead man for travel, Odilia walks along the river and encounters La Llorona. Also known as the Weeping Woman in Latin American folklore, she is a spectral mother searching endlessly for her drowned children. La Llorona informs Odilia that she and her siblings have been chosen to return the dead man to his family and that their journey will help Odilia’s family find one another again. She gifts Odilia an Aztec ear pendant with magical properties that will provide the girls with safe passage five times. Juanita does not believe Odilia when she tells her about La Llorona, but Odilia puts the pendant in her ear anyway.
The girls position the dead man in the backseat of Papá’s old car and pretend he is asleep while driving across the border. In a moment of panic, Odilia uses the ear pendant to hypnotize the border official, calling upon the Aztec mother goddess Tonantzin. They pass into Mexico undiscovered and make their way to El Sacrificio, constantly bickering. At one point, Velia and Delia tie Pita up to prevent her from running away to call Mamá and tattle, and Odilia cannot imagine this journey uniting them like La Llorona claims. When they arrive at Gabriel Pérdido’s house, they interrupt his daughter Beatriz’s quinceañera. Here, the Garzas learn that the dead man—like their own Papá—abandoned his wife and children long ago, making his posthumous return even more devastating. Having ruined the party and feeling gloomy about Gabriel and Papá’s actions, the girls depart before the dead man’s family notices that the news stations are reporting all five sisters as missing children.
Papá’s car breaks down soon after the sisters leave El Sacrificio, so they continue in the direction of Abuelita’s house on foot. Soon, they come upon a beautiful widow named Cecilia. She welcomes them into her enormous home, feeding them sweets until they are too drowsy to stand up. Odilia calls for La Llorona’s help, and the specter brings a concoction that purges Cecilia’s medicated food from her system. With La Llorona’s help, Odilia destroys Cecilia’s pies, which are laced with a drug that will keep the girls as docile pets forever. She then rallies her sisters to fight the sorceress. Using the ear pendant, Odilia forces Cecilia to help them, and she directs them to the house of Teresita, a seer. Looking back, they see that Cecilia’s house is a ruin and Cecilia herself is an extremely old woman. The twins mock her, and Cecilia calls upon her “evil children” to pursue them.
At Teresita’s house, the seer tells the girls they will have to pay for their poor treatment of Cecilia. She predicts three dangerous encounters with her “evil children”—the girls will meet a shapeshifting warlock called the nagual, witch owls called lechuzas, and the monstrous beast known as el chupacabras. Odilia’s sisters are skeptical of Teresita’s claims. Heading toward Abuelita’s on foot, they encounter the nagual disguised as a kindly donkey, and they later wake in his cave. Calling upon Tonantzin with the ear pendant, Odilia summons the goddess herself, who tells the nagual the girls are under her protection. Taking refuge in an abandoned barn, the sisters encounter the lechuzas next; the witch owls shriek in stolen voices about what wicked children they are. By working together, the girls eventually defeat them. Finally, they befriend a young goatherd named Chencho only to discover he transforms into the chupacabras at night. Although Chencho bites Pita, Odilia insists they show him mercy, seeing that he is only a boy and determined to remain “pure of heart” as La Llorona instructed.
Odilia and her sisters reach Abuelita Remedios’s house just in time to save Pita, whose wound is infected. Luckily, Abuelita is a skilled herbal healer. She is overjoyed to see the girls and invites them to stay while Pita recovers. While they are there, Abuelita reveals that Papá is alive but that he is trying to divorce Mamá for selfish reasons. She is adamant that the girls are not at fault for his abandonment. Abuelita drives the Garza sisters back to the border, at which point they realize they have forgotten their birth certificates. Odilia uses the ear pendant’s last spin to wish to go home, and they encounter Tonantzin once again. The goddess congratulates them on their success and asks for one more favor—to remind “the mother” who she is by giving her special roses they will find in the woods. She then gifts them a vision of the ancient Aztec city of Tenochtitlan before returning them safely to shore, where the roses wait.
After an emotional reunion with Mamá in the customs office, the girls return home to find Papá there. Although her sisters embrace him, Odilia senses that he is not there for them and holds back. Moments later, it is revealed that Papá has brought his new wife-to-be and stepdaughters to live in the house, intending to kick Mamá out of her own home. Outraged by this treatment of Mamá, who they now appreciate more than ever, the girls reject Papá, demanding he leave. Later, Odilia realizes that Tonantzin intended the roses she picked for La Llorona, the mother in need of a transformation. Accepting them, La Llorona’s journey of grief ends and she is reunited with her children among the stars. Tonantzin leaves Odilia with the knowledge that she is never alone, as her ancestors are always with her.
Many months later, Mamá throws Odilia a belated quinceañera. Papá shows up, left destitute by his new wife, but he finds that Mamá has moved on in both love and life. Odilia is cordial with Papá but remains distant, wondering if they will ever be close again. Watching her sisters and Mamá, so full of love and joy, she feels certain that they will always be together.
