Plot
Three years after saving Winter, the staff at Clearwater Marine Aquarium rescue a severely sunburned beached dolphin, named Mandy after a little girl who finds her. A now 14-year-old Sawyer is offered a scholarship to the elite SEA Semester program, which involves three months at sea doing marine mammal research, after impressing his cousin Kyle's advisor Dr. Miguel Arroyo when champion surfer Bethany Hamilton swims with him and Winter. Sawyer is reluctant to go because he is concerned about leaving Winter.
Winter needs attention and shows signs of stress and loneliness, especially after her older companion, Panama, dies from old age. Acting on Rufus the pelican’s erratic behavior one morning, Sawyer and Hazel find and rescue a sea turtle from some fishing line. Rufus becomes obsessed with the sea turtle, named Mavis, to the point he even watches her during a CT scan at the hospital where Lorraine works.
Meanwhile, George Hatton, a USDA inspector, insists that Winter must be matched with a new companion within 30 days or be transferred elsewhere. However, Winter's behavior has become unpredictable, even dangerous; when Sawyer tries to get her out of the tank to prepare to make her a new prosthetic tail, she becomes aggressive and knocks him into the water, injuring him. Dr. Clay prohibits Sawyer from swimming with Winter until further notice. Winter spends weeks in isolation, and despite people not being allowed to visit her, Sawyer allows Susie, a new volunteer, and her grandfather to see her. Hazel, who has feelings for Sawyer, becomes slightly jealous due to Sawyer getting attention from another girl.
Sawyer and Hazel hope Mandy will become Winter's companion. Mandy's rehabilitation at CMA goes well, so Clay contemplates releasing her soon, causing alarm for Hazel in particular. Lorraine advises Hazel that no one spends any more time at a hospital than they need to and encourages her to talk with her father. Hazel assertively yet respectfully asks Clay to let her view Mandy's medical records, which he does. Clay, Sawyer, and Hazel test Mandy's readiness by giving her three live fish, which she catches effortlessly, so they reluctantly decide they must release her. Clay puts Hazel in charge of her release.
Sawyer and Hazel break the rules and swim with Winter, who appears to be feeling better. Clay finds them doing so and scolds them, but then states that it does not matter, because George issued an order that Winter be transferred to a marine park in Texas. Meanwhile, Lorraine and Sawyer's friends arrange a going-away party for Sawyer's anticipated participation in SEA Semester, although he still cannot decide whether to go. Dr. McCarthy shows him an old watch which must be tapped to keep ticking, and so encourages Sawyer to "shake it up now and then" and try new things in life.
During the party, Clay is informed that another female dolphin has been rescued and is headed to CMA; one so young that she has not yet learned how to hunt and cannot return to the wild. This offers new hope of companionship for Winter, so they name her "Hope". Meanwhile, Mavis is released, and Rufus follows her out to sea.
George gives Clay an extension to keep Winter from being transferred, and CMA tries to introduce Hope as her new companion. When they attempt to introduce the two, Hope panics and circles the pool at high speed, forcing staff members to quickly separate the two dolphins. They realize that Hope had sensed that Winter had no tail and moved differently from other dolphins. Sawyer thinks that a new prosthetic tail might have better results; though Phoebe, Rebecca, and Kat are initially wary of the potential repercussions, everyone agrees.
In her second meeting with Hope, Winter wears the new tail designed by Dr. McCarthy. A huge crowd, including George, is present to watch, and eventually the two dolphins accept each other. Everyone rejoices that Winter can stay at CMA now that she has a companion. With his uncertainty about Winter resolved, Sawyer decides to go to the SEA Semester program, but reassures her that he will be back. Hazel says goodbye to him by releasing helium balloons and holding up a sign, and Sawyer sends her a text message saying, "Thanks, Hazel. You're the best!", causing her to blush as she smiles to herself, while Rufus returns to the aquarium. As Sawyer, Kyle and Lorraine drive away, Hazel quietly affirms she will really miss Sawyer.
Real-life footage shown at the end includes Mandy's actual release, Hope's rescue, a sea turtle named Mavis and her release, Winter wearing the new prosthetic tail, and amputees interacting with Winter.
