Whirligig vividly illustrates how actions, whether good or bad, large or small, perpetuate in endless cycles of consequence, much like the ripples emanating from a pebble dropped into water.

Brent Bishop, a sixteen-year-old junior at Montfort, a private high school in Chicago, seeks self-identity and peer acceptance through the right clothes, car, school, money, alcohol, and a girlfriend. His quest leads him to a party with a friend who fails to mention it's a Chess-themed event where everyone must dress in all white or black. "He is always the new kid, navigating the maze, never quite wealthy, attractive, or athletic enough to join the elite. Tonight's party is his chance to change that."

Frequent relocations, as his father climbs the corporate ladder, amplify Brent's feelings of isolation and insecurity, leading him to make poor choices. Alcohol and pot are readily available at the party, and Brent, "proud of his ability to handle hard liquor," drinks heavily. When Brianna, the girl he aims to impress, verbally "slaps him in the face," he loses control, punches his host, and leaves the party in shame and despair. Lost on the wrong expressway, he spirals into self-pity. Overwhelmed by the humiliation he expects from his classmates on Monday, he decides on suicide, taking his hands off the wheel. Instead of ending his own life, he tragically ends the life of eighteen-year-old Lea Zamora. She was a "senior at Niles North High School, an honor student, student council member, orchestra participant, track team athlete, active in the Filipino community, and a volunteer at Resurrection Hospital. Why did he have to kill someone like her?"

At the accident scene, Brent's driver's license is confiscated, and during a hearing, the judge sentences him to probation in a detention center. Brent initially feels relieved, then dissatisfied with the ruling, realizing he craved a harsher punishment. When he meets with Mrs. Zamora "to apologize, and to understand, and to atone," she tells him she does not believe in retribution. She shares Lea's love for life, her ability to make people laugh, and her passion for whirligigs. Mrs. Zamora requests, "that [he] make four whirligigs, of a girl that looks like Lea. Put her name on them. Then set them up in Washington, California, Florida, and Maine—the corners of the United States. Let people all over the country receive joy from her even though she's gone. You make the smiles that she would have made. It's the only thing you can do for me. That's what I ask."

Mrs. Zamora provides Brent with a Greyhound bus pass valid for forty-five days. Despite his parents' objections, Brent agrees to undertake the journey. Armed with plywood pieces, new tools, an old book on building whirligigs, and some camping gear in his backpack, he embarks on his trip to Washington. He visits each state Mrs. Zamora specified, crafting increasingly intricate whirligigs based on patterns from the book. Throughout his travels, Brent's self-esteem improves. He starts to appreciate simple pleasures, develops a newfound interest in reading, and enjoys learning new skills. He feels a deeper connection to the world around him, shedding his sense of isolation and gaining a sense of self-worth. Brent transforms from a self-centered, somewhat unlikeable young man into a more mature and likeable individual.

Interspersed chapters flash forward in reverse chronological order, revealing the significant impact the whirligigs have on the lives of others who encounter them. These individuals include an overly studious eighth-grader and her best friend in Maine, an immigrant from Puerto Rico sweeping streets in Miami, Florida, an adopted Korean boy in Washington, and a dying grandmother with her teenage granddaughter in San Diego, California.

Upon completing the final whirligig and installing it at an artist's home in Maine, Brent experiences a sense of transformation, feeling unusually light-hearted. "The guilt hasn't magically vanished overnight. Four whirligigs wouldn't accomplish that." However, he feels ready to return to Chicago, reunite with his parents, deliver the photos to Lea's mother, and start at a new school in the fall. "He felt that he was up to it."