Sawyer and his older cousin Kyle (Austin Stowell), watch a swim meet. Kyle speaks to his former coach, (Russ Blackwell) about Kyle's impressive swim records and he talks of going to the Olympics. This upsets Sawyer because the only way to pay for training is for Kyle to deploy to the military. Later on the boys are at Kyle's going away party and Sawyer stays distant. His mother talks to a neighbor explaining that Sawyer's academic performance in school is poor therefore he is required to attend summer school to raise his grades. Kyle finds Sawyer in his workshop inside the garage to say his goodbyes and gives him a pocket knife. The next day Sawyer watches Kyle leave and heads to school on his bike.Sawyer Nelson (Nathan Gamble) is biking along the beach when a fisherman (Richard Libertini), calls for help after finding an injured bottle-nose dolphin tangled in a crab trap. The two call for assistance, and rescue workers from the Clearwater Marine Hospital, run by Dr. Clay Haskett (Harry Connick Jr.), take the injured dolphin for treatment. Clay's daughter Hazel (Cozi Zuehlsdorff) names the dolphin Winter, as two prior dolphins, Summer and Autumn, were successfully returned to the ocean, and believes that using seasons as names will continue the streak. She allows Sawyer to see Winter.Clay initially does not like the arrangement since Sawyer is not trained in marine animal care. She notices that Winter responds well whenever Sawyer is around, and he is allowed to visit. Soon Sawyer, who was enrolled in summer school due to his nearly failing all his classes during the year, starts skipping classes daily to visit Winter. Sawyer's mother Lorraine (Ashley Judd) finds out about Sawyer skipping classes, but after seeing that Sawyer's interaction with Winter has improved his moods and well-being. This is something Sawyer had not shown since being abandoned by his father who disappeared five years earlier. Lorraine withdraws him from summer school and agrees to let him to volunteer at the hospital.Unfortunately, Winter's tail is damaged and thus must be amputated. Winter learns to swim without a tail by developing a side-to-side motion, like a fish, but after an x-ray Clay notices that the unnatural motion is causing stress on her spine; if continued the motion will eventually kill Winter. Meanwhile, Sawyer's cousin Kyle (Austin Stowell), a champion swimmer, returns from the military with a damaged right leg from an explosion. Sawyer wants Kyle to meet Winter and excitedly anticipates seeing him at a welcome-back party thrown by his aunt but is devastated to learn that Kyle has skipped the party, preferring to stay at the local Department of Veterans Affairs Medical Center.Sawyer and Lorraine go to visit Kyle who is working with Dr. Cameron McCarthy (Morgan Freeman) in the facility's prosthetic lab. To Sawyer's surprise, Kyle is embarrassed to see them and even asks them to leave, which infuriates Sawyer. Kyle takes him on a walk and talks to Sawyer about his leg. Sawyer then asks Dr. McCarthy about a prosthetic tail for Winter. He agrees to work on the project during his upcoming vacation and convinces his prosthetic supplier (Hanger Prosthetics and Orthotics, which supplies Winter's real-life tails) to supply the parts at no cost. Dr. McCarthy manufactures a "homemade" model tail while waiting for the real one to arrive; however, Winter destroys it by banging it against the pool wall.Shortly thereafter the hospital, already in financial peril, is seriously damaged by Hurricane LeRoy, whereupon the board of directors agree to close the hospital, sell the land to a real estate developer, and find homes for the animals, except Winter, who due to her condition is not wanted by anyone and may have to be euthanize. However, after a chance encounter with a mother and daughter (who also had a prosthetic limb) who heard about Winter's story and drove all the way from Atlanta to see her, Sawyer comes up with a last chance plan ("Save Winter Day") to save the facility. Clay is not sold on the idea, but reconsiders after talking with his father, Reed (Kris Kristofferson). Kyle agrees to a race against Donovan Peck, the current local swim champion who followed him at high school and broke nearly all his prior swim records and enlists a female friend at Bay News 9 to promote the event.The Hanger-supplied tail finally arrives; however, Winter damages it as well. Sawyer discovers that Winter isn't rejecting the tail; instead, the plastic to which the tail is attached is irritating her skin, which Sawyer compares to a seat belt chafing human skin. Dr. McCarthy comes up with an alternative gel-like sock (which he calls "Winter's Gel", the real-life name of the Hanger product used to attach prosthetic limbs, which was developed during its research with Winter). When Winter is fitted with the new prosthetic tail, and she is then able to accept the new plastic tail.At Save Winter Day, Winter is released into a river for the event and Sawyer's former summer schoolteacher gives him credit for his work at the hospital, allowing Sawyer to pass summer school despite not attending formal classes. The fisherman who initially spotted Winter places $40 in the donation jar. The board learns that the real estate deal has closed; however, the developer, who attended the event with his grandchildren, agrees to allow the hospital to remain open and says he will financially support it. Kyle and Donovan Peck then race in the river, which Kyle then wins (with the help of Winter). Sawyer then dives into the river with Winter.The ending shows footage from Winter's actual rescue and that she is happy and healthy. It then shows several of the prosthetic tails that Winter has worn, and scenes from real amputees who have visited Winter at the Clearwater Marine Aquarium.