Cordy, a red-haired, gray-eyed middle schooler, lives with her mother, Casey, and younger brother, Geno, in a run-down hundred-year-old boathouse on the Oregon coast. The boathouse sits on the property of the McKern family, wealthy Californians who visit only a few times a year. Casey serves as their unpaid property manager in exchange for free rent and utilities.
Two years and four months earlier, the family lived in a rental house in nearby Dunes City, and Cordy's father was a commercial crab fisherman. Then came what the family calls The Accident: Early one April morning, his boat hit a sandbar, and a sneaker wave, a large unexpected wave, knocked him into freezing water and killed him. With no savings or life insurance, Casey quit her nursing classes. Extended family scattered, and the family eventually landed in the boathouse through Casey's friend Taffy. Cordy's father once called her a Daughter of the Sea, but since his death she has been afraid of the ocean.
One afternoon, Cordy finds a thin, dirty dog following her home from school. She feeds him, carries him home, and bathes him. Casey agrees to help find the owner, and they post LOST DOG flyers around town. Geno notices the biggest word on the flyer is "LOST," and the family begins calling the dog by that name. No one claims him, and after a month Casey quietly accepts Lost into the family.
One Saturday morning, Lost disappears. Cordy hears his barking from the McKern dock on the Siuslaw River and runs down to find him barking at a pod of bottlenose dolphins that have entered the river from the ocean. Among them, she spots a small pink dolphin, an albino whose coloring results from a rare gene mutation that prevents melanin production. She shrieks with excitement, momentarily forgetting her fear of the water. Afterward, she reflects that she wants to tell her father about the dolphins, about Lost, and about standing on the dock unafraid.
Cordy researches dolphins at the Siuslaw Public Library with help from Mrs. Hunt, the reference librarian. While Cordy reads, Geno wanders off, sending her into a panic until she finds him inside a meeting room where a community services presentation offered free cookies. Mrs. Hunt gives Cordy a backpack she claims came from the lost and found; Cordy later discovers a receipt inside bearing Mrs. Hunt's name and realizes the librarian gave away her own backpack.
Cordy grows worried about Lost's persistent bad breath, which can signal serious health problems. Geno produces a brochure from the community services presentation: A free veterinary clinic operates monthly at the humane society for low-income community members. Cordy persuades Casey to let her take Lost.
At the clinic, Cordy is overwhelmed by intake paperwork and nearly leaves, but Daria Obermeier, a woman she recognizes from the local boat ramp, lets her skip the forms. The volunteer veterinarian, Dr. Mazari, examines Lost and diagnoses rotten teeth and infected gums requiring dental surgery estimated at $1,200 to $2,500. Overwhelmed, Cordy breaks down crying. Dr. Mazari calls Casey at Curly's Seafood, the restaurant where Casey works, and proposes performing the surgery at Oregon State University's (OSU) veterinary program as a teaching case at no charge. Casey arrives and exchanges phone numbers with Dr. Mazari, visibly upbeat.
That night, Cordy wakes to find Casey on the couch, smiling and texting, behavior completely out of character. Casey announces that "Taj," Dr. Mazari's first name, has scheduled Lost's surgery. He offers to drive to Florence to draw Lost's blood, and Casey invites him to dinner in return. Alarmed by their growing connection, Cordy wears her father's oversized green hoodie to the dinner as protest while Casey gets a new haircut and borrows a sweater for the occasion.
Dr. Mazari brings thoughtful gifts: a book of heroes for Geno and a book about dolphins for Cordy. At dinner, Geno surprises everyone by speaking Spanish with Dr. Mazari, a skill Geno picked up during playdates at his best friend Jose's house. When Dr. Mazari reveals both his parents have passed away and that he moved to Oregon for a fresh start, Cordy notices Casey's empathetic response and feels threatened. When Casey suggests skipping dessert, Cordy reaches her breaking point: She removes her father's hoodie and leaves the restaurant in tears.
Seeking comfort, Cordy visits Valerie Crowley, an elderly neighbor who lives alone. Valerie quotes psychologist Carl Jung: "The meeting of two personalities is like the contact of two chemical substances: If there is any reaction, both are transformed" (140). She tells Cordy that Casey deserves to be happy and gives Cordy a Polaroid camera with expired instant film that produces ghostly, faded images.
Casey and Cordy leave before dawn for Lost's surgery at OSU in Corvallis. While waiting, they explore the campus together. Casey reflects that Cordy's father was smart enough to have attended a university but never had the chance. It begins to rain, and Casey takes Cordy's hand, something they have not done since the father's memorial. After Dr. Mazari reports the surgery went well, they meet him for lunch at a campus dining hall. Cordy photographs Casey and Dr. Mazari on the steps outside; the expired film makes them look as if they are glowing. On the drive home, Cordy tells Casey that she had a dad and Casey had a husband. After a long silence, they roll down the windows and sing "Two of Us" by The Beatles, a song Cordy and her father always played on his boat. That night, Cordy slips the Polaroid into a book on her shelf, a quiet, private act.
School lets out, and Cordy's best friend, Button, announces that her family is moving to California for competitive swim programs. Cordy is devastated.
Dr. Mazari begins visiting Florence regularly. One Saturday, they all drive to Heceta Head State Park. On the beach, Lost's barking leads Cordy to a tidepool where she discovers an octopus trapped inside a discarded plastic water bottle. Dr. Mazari immediately climbs into the deep tidepool fully clothed, cutting his hands on rocks to retrieve the bottle. He slices it open, and they wade into the surf to release the octopus. Cordy realizes she is standing in the Pacific Ocean for the first time since The Accident. When Dr. Mazari leaves that evening, she thanks him sincerely, and for the first time, she does not want him to go. That night, she retrieves the Polaroid and places it against her mother's bedside lamp, a gesture of acceptance.
In a time jump, Cordy reveals that the McKerns sold their property. With Taffy's help, Casey and Taj bought the Big House. The boathouse has been repurposed as a wildlife rehabilitation center. Casey completed her nursing degree and is training to become a physician's assistant. Cordy and Geno join Taj on weekend animal rescues, and Cordy is no longer afraid of the ocean. Geno was the first to stop using the word "stepfather," introducing Casey and Taj simply as his parents. Cordy reflects on the chain of events: Finding Lost led her to the dolphins; curiosity led her to the library; Geno's wandering led to the free clinic brochure; and the clinic led to Taj. She may never see the pink dolphin again, but knowing it is out there makes her feel strong. She honors her father's memory while embracing the family Taj has joined, concluding that she had a great father, she exists because of him, and that is enough.