Sawyer and his older cousin, Kyle, find themselves at a swimming competition where Kyle discusses his aspirations of participating in the Olympics with his former coach, Russ Blackwell. This conversation disturbs Sawyer deeply, as the only way for Kyle to finance his training is by enlisting in the military. Tension builds at Kyle's going-away party; Sawyer keeps his distance while his mother confides in a neighbor about Sawyer's struggles in school, leading to his enrollment in summer school.
As Sawyer gears up for school on his bike, he encounters a fisherman, Richard Libertini, who desperately seeks help for an injured bottlenose dolphin caught in a crab trap. Rushed to the Clearwater Marine Hospital, the dolphin is treated by Dr. Clay Haskett (played by Harry Connick Jr.), while his daughter, Hazel, names the dolphin Winter, hoping that this name will continue a streak of successful rescues. Though initially reluctant, Clay allows Sawyer to visit Winter after observing their special bond.
Despite having to attend summer school after barely passing his classes, Sawyer finds himself skipping school to spend time with Winter. His mother, Lorraine, discovers his truancy but recognizes the positive impact Winter has on Sawyer's mood, which has suffered ever since his father left five years ago. Consequently, she decides to withdraw him from summer school, permitting him to volunteer at the aquarium instead.
Tragedy strikes when Winter’s tail is severely damaged and has to be amputated. Adapting to her new reality, Winter learns to swim using a side-to-side motion, but this unnatural technique jeopardizes her health. Meanwhile, Kyle returns from military service with a severe injury to his leg. Excited for their reunion, Sawyer is heartbroken when Kyle avoids attending the welcome-back gathering, choosing instead to focus on his recovery at the Department of Veterans Affairs Medical Center. Determined to help Winter, Sawyer seeks guidance from Dr. Cameron McCarthy, who is working closely with Kyle.
In a twist of fate, Dr. McCarthy agrees to develop a custom prosthetic tail for Winter as he prepares to leave for vacation. However, while he constructs a temporary model, Winter destroys it in the pool. The Clearwater Marine Hospital faces grave financial issues, compounded by the devastating effects of Hurricane LeRoy, leading the board to consider closing the facility. With Winter facing euthanasia, Sawyer is inspired after witnessing a mother and daughter, both with prosthetic limbs, visiting Winter, and he hatches a plan named "Save Winter Day" to rally support for the hospital.
To gain attention, Kyle contemplates competing against his former rival, Donovan Peck, and enlists the help of a local news outlet. The anticipated day arrives, bringing people together for the cause. Though Winter initially rejects the new prosthetic tail, Sawyer discovers that the irritation is due to the material used. Dr. McCarthy develops an alternative solution, allowing Winter to finally accept the new tail.
On the day of the event, Winter is released into the river, and Sawyer's effort to help at the hospital pays off; he is given credit for his contributions, allowing him to pass summer school despite not attending formal classes. Donations pour in to keep the hospital afloat, culminating in a heartwarming race between Kyle and Donovan, where Winter’s support helps Kyle emerge victorious. The film concludes with real footage of Winter's rescue, showcasing her healthy and joyful life, accompanied by glimpses of other amputees who have found inspiration in her story at the Clearwater Marine Aquarium.
