The following version of the book was used to create this study guide: Dillon, Alena. The Happiest Girl in the World. HarperCollins Publishers, New York, NY, 2021. Kindle AZW file.

When Charlene Wheeler was a young mother with twins – Sera and Joe – she was happy to take advantage of classes at the local YMCA for her children. While Joe was in basketball, Sera was learning gymnastics. One day, the gymnastics teacher told Charlene that Sera had talent that would never be fully realized in the local class. Charlene was ready to dismiss the idea of a more advanced class. She became enamored with the idea that Sera might someday be a national, world, or even Olympic champion. The family began to sacrifice as Charlene drove Sera an hour away for practice. Eventually, this became their daily commute. Sera became close friends with another gymnast, Lucy, and the girls set their sights on the Olympics.

Sera and Lucy train through all sorts of brutal conditions. They support each other through physical and emotional pain. They have support in the form of coaches and officials, including Jennifer who eventually agrees to take on training Sera and Lucy full time. Both girls stop attending school to train 40 hours a week under one coach, Jennifer. They attend a camp operated by former gymnast Vanda Balogh. A doctor named Eddie Levett takes care of the medical needs of the gymnasts. One evening, Eddie slides his hand under Lucy's nightgown. Neither Lucy nor Sera immediately realize what is happening. They learn that he is touching many others in the same way. When Lucy makes an outright accusation against Eddie, Vanda is furious. Sera, fearing she will be cut from consideration for the quickly-approaching 2016 Olympics, does not support Lucy's claim. Lucy drops out of gymnastics altogether, and Sera is not chosen for the Olympic team.

Sera stays at home and tries to settle into the normal life of a teenager. She quickly realizes that she is not happy without gymnastics. Though Charlene supports Sera's decision to leave gymnastics, Charlene has floundered. Sera's father (Bob) and brother (Joe) think about all the sacrifices they have made as a family. Jennifer refuses to train Sera, citing Sera's age and the brutal workouts required leading up to the 2020 Olympics. A coach named Lou Gently agrees to take on Sera. He designs a regimen that will put Sera in peak condition for the Olympics. During the Worlds competition, Sera ruptures her Achilles tendon which sets her back months. With the end finally in sight, COVID strikes. Lou helps set up make-shift equipment at Sera's house and closes his gym. Sera's family “hunkers down” to wait out the pandemic. Olympics are postponed a year.

Sera performs beautifully at the Olympic Trials, but she and Lou clash over how far they are willing to go for a victory. Sera is happy with what she has accomplished, but she knows there have been a lot of sacrifices to reach this point. As the novel comes to an end, Sera watches herself and her mother on the Jumbotron monitor as Sera takes her place as one of four American gymnasts who will participate in the Olympics.