Fritz Robinson is the eldest son, about fifteen, from a middle-class Swiss family. His father is a doctor and his mother manages the home. Aspiring to be a music composer and skilled with the flute and guitar, Fritz initially resists the family's permanent move to Australia, fearing limited musical opportunities. Though arrangements are made for him to stay in Switzerland with relatives, he changes his mind last-minute and joins the voyage.

Aboard the Blackburn Rock, Fritz befriends Emily, a young English passenger. When a catastrophic storm wrecks the ship on an uninhabited island, Fritz survives lashed to floating wreckage by the captain. He reunites with his parents and siblings, who reached shore on a makeshift raft. On the island, Fritz assists his father in exploration, carpentry, and survival. He helps build shelters, including a treetop house due to jackal threats, and later aids in constructing a seaworthy vessel for escape. His duties include hunting, gathering resources, and protecting the family from dangers like wild animals and earthquakes.

Fritz evolves from a reserved, music-focused youth into a more resilient and proactive figure. His relationship with Emily, met during the voyage and later reunited with in Australia, hints at romance. Interactions with the abrasive Captain Morton, who steals family supplies, foster Fritz's wariness of outsiders. However, he demonstrates loyalty to his family, friends, and the Aboriginal boy TomTom. After over a year on the island, Fritz participates in the perilous voyage to Australia. Upon arrival, he witnesses Emily's decision to study nursing in England. He remains in Australia with his parents and younger brothers as Emily, Morton, and TomTom depart.