Michael is a ten-year-old boy whose parents lose their jobs, leading the family to buy a yacht named the Peggy Sue and sail around the world. He is highly observant, keeping a detailed ship's log of his family's maritime journey and drawing the wildlife he sees. After falling overboard with his dog during a storm, he washes up on an isolated island where he must rely on his resourcefulness to survive.

Dr. Kensuke Ogawa is an elderly, emaciated Japanese man and the sole human inhabitant of the island where Michael washes ashore. Formerly an obstetrician and a doctor in the Japanese Navy during World War II, he has lived alone for decades. He possesses profound survival skills, foraging for food and creating tools, while maintaining strict territorial rules for his home.

Michael's mother and father respond to sudden unemployment by purchasing a 42-foot yacht, the Peggy Sue, and deciding to travel the world. The father acts on bold ideas to keep his family's spirits high, while the mother methodically studies to earn her Yachtmaster's certificate. Together, they operate as a cohesive team to face the intense challenges of the open ocean.

The poachers are a group of unnamed hunters who travel on a Chinese junk with red-brown sails. They visit the island to hunt gibbons and steal their babies for profit. Their presence introduces a violent threat to the otherwise peaceful ecosystem of the island.

Stella Artois is the family's one-ear-up, one-ear-down black-and-white sheepdog. She possesses a seemingly prescient ability to sense trouble before it happens. At sea, she provides comfort during storms, and on the island, her energetic presence helps ease Michael's intense isolation.

Eddie Dodds is Michael's best friend from his life in England. When Eddie moves away, he gifts Michael a soccer ball as a parting present. This object becomes Michael's most treasured possession and a talisman of good luck throughout his journey.

Michiya Ogawa is Kensuke's son, whom Kensuke left behind in Nagasaki when he was drafted as a doctor in the Japanese Navy. Kensuke holds onto memories of Michiya, painting his portrait from memory inside the cave to keep his connection to his family alive.

Kimi is Kensuke's wife. She lived in Nagasaki with their son, Michiya, during World War II. Kensuke frequently thinks of her during his decades of isolation on the island, though he struggles with the fading memory of her face.

Tomodachi is a mother orangutan and Kensuke's closest companion among the island's wildlife. She frequently visits his cave and relies on him for safety when threats approach the island.

Bill Parker is an experienced mariner who trains Michael's parents to sail. He provides the instruction necessary for the family to safely operate the Peggy Sue before they set off on their global voyage.

Uncle John is Michael's relative in Australia. The family spends nearly six weeks visiting him and his children, taking a break from their ocean voyage to enjoy life on land.