The following version of the book was used to create this study guide: Medina, Margaret. Merci Suarez Changes Gears. Candlewick Press, Somerville, Massachusetts, 2018. Kindle AZW file.

Mercedes Suarez, known as Merci to her friends and family, is just beginning sixth grade. She is not certain she wants to keep up with the rapid changes taking place in her life. Her life at school has changed because the girls and boys no longer sit together at lunch and there is a new tension between them, especially when a new boy named Michael arrives from Minnesota. Merci and her friends are now changing classes, meaning they do not have that close relationship she had with her fifth-grade teacher. Her brother Roli is spending all his time focused on his senior classes, his plans to attend college in the fall, and practicing driving. Her young twin cousins have started school, and Merci mainly sees them as being always in the way. But, the biggest change of all is that her beloved grandfather, Lolo, is not himself. Merci is furious when Roli finally tells her that Alzheimer's is wrecking Lolo's brain and that the disease will progressively take his memories and his personality.

At Seaward Academy, Merci is part of a group that includes Edna, Lena, Rachel, and Hannah. Merci always feels like something of an outsider and has a grudging admiration for Edna, the leader of the group. When Edna wants things done, she uses her “mojo” to make them happen. When Edna sets her sights on Michael, she is angry that Merci is assigned to be Michael's Sunshine Buddy – a group of students who are supposed to make new students feel welcome as they adjust to the school. Merci struggles to fulfill her duty to be Michael's friend while trying to avoid being labeled as anything more than a friend. Meanwhile, Edna goes out of her way to berate Merci and anyone else who catches her attention. Their situation comes to a head when a school official discovers it was Edna who destroyed the costume Merci's grandmother helped make for Michael. While Edna must formally apologize, Merci realizes that she can sometimes be rude as well. The girls may never be friends, but they seem to agree to be nicer to each other. Merci, meanwhile, discovers that Lena and Hannah are kind and honorable, just the kinds of girls who can be true friends at a time when Merci needs them most.

The novel comes to an end during the Christmas holiday from school. Merci spends a day with her family and her father's employee, Simon, who seems to be developing a romantic interest with Tia Inez. Merci spent a lot of time and effort creating a photo scrap book, her gift to Lolo though she says it is really a gift for the entire family. She has placed photos taken over the past few weeks of all the family members, leaving room for additional photos in the coming months. She plans to use them to help Lolo when he struggles to remember. Merci's gifts include a new bicycle to replace the old one that has become the bane of her existence. That night, while everyone else is sleeping, she rides around the yards of the three houses occupied by her family. She changes gears on the bicycle, feeling the immediate response in her pedaling, and realizes that change is a necessary part of life. Though she dreads the changes that will happen to Lolo, she realizes that change can also be good.