Twelve-year-old Natalie Perkins is a high-energy, imaginative girl who feels out of place at school and at home. After a chaotic field trip to the Smartwater Power Plant, an experimental facility combining nuclear and hydroelectric technologies, Natalie falls into a contaminated holding tank and is rescued by paramedics who deliver massive electric shocks. The ordeal triggers unusual changes: she begins to "sync" with others' emotions and perceptions, experiences prophetic flashes, emits electrical jolts when angered, and develops a luminous inner energy. A talking frog named Quincy appears in her visions and helps her understand emerging abilities; Natalie adopts the superhero identity "Ultra" in her imagination.
At the plant, ambitious owner Albert Smart and his security chief, Mr. Cain, are concealing unsafe practices: irradiated water is being stored and secretly reintroduced into the system to save money. Walt Rutherford, an engineer who began to investigate the missing radioactive water, is fired, pushed out by the plant's management. Later, Dr. Walter Rutherford surfaces as the school custodian, quietly monitoring the plant and conducting his own drone-based investigations. He discovers evidence suggesting illegal disposal and reintroduction of radioactive water. When Natalie's accident implicates the plant, Mr. Smart and his men attempt to silence any threats.
Natalie's powers attract danger. Two hired thugs, Hector and Armand, repeatedly try to capture her, with each attempt ending badly for them. Natalie's abilities repel or incapacitate them, sometimes in ways she can't fully explain. With help from her best friend Harrison Parker, a brilliant and inventive boy, and an unlikely ally in Martin Chubinski, the big, lonely classmate, Natalie begins to channel her gifts more deliberately. Harrison builds gadgetry, like The Cloak (a fiber-optic camouflage) and The Lifeline (an elastic rescue rope) to protect and aid her; Dr. Rutherford uses them in a daring plan to infiltrate the plant and retrieve incriminating files.
The plan: Natalie's family attends a goodwill tour at the plant while Harrison and Dr. Rutherford use The Cloak and Lifeline to access the roof and the servers. The mission initially succeeds: Dr. Rutherford downloads files proving illegal waste handling. Mr. Cain and the thugs pursue, but clever improvisations, like the school's powerful vacuum used as a trap, capture Armand, and a drone nets Hector. When Mr. Smart himself confronts Natalie, she is abducted and taken to a hidden shed containing barrels of contaminated water. There, pressed to the limit, Natalie consciously separates her calmer, "good" self from an energized form and unleashes a controlled, enormous burst of energy, "going nuclear", to destroy the plant's illicit cache and escape. The explosion incapacitates Smart and exposes his crimes.
In the aftermath, authorities arrest the perpetrators, the plant's board replaces leadership with Dr. Rutherford, and the town grapples with contamination and scandal. Natalie recovers in hospital, keeps her full powers largely secret, and returns to school with new confidence and purpose. Harrison is honored as Student of the Year; Martin gains respect; the community recognizes Dr. Rutherford's heroism. Natalie's journey from misfit to protector closes the book on an optimistic note: she's different, but her unusual gifts, her "Ultra" energy, allow her to protect others, belong, and grow.