Franz Robinson, the family's eldest son, aspires to become a musician. He initially opposes relocating to Australia, fearing limited musical education opportunities. Changing his mind at the last moment, he boards the Blackburn Rock, befriending a passenger named Emily. During a devastating storm and shipwreck, the captain secures Franz to a wooden log for survival while the rest of his family escapes on a raft.

Stranded on an uninhabited island, Franz aids his father, Dr. Ernest Robinson, in vital exploration and carpentry tasks like building shelters and boats. He shoulders responsibility through daily chores and continued studies. Franz distrusts the injured ship captain, Mr. Morton, whom they encounter, disapproving of Morton's actions, including stealing supplies from their treehouse.

Learning the island is volcanic, Franz actively participates in constructing a boat for their escape to Australia. Arriving north of Sydney, the family travels by train to Melbourne. There, Franz reunites with Emily, now training as a nurse. Emily later departs for England to pursue advanced medical studies. Franz's long-term plans following this reunion remain unspecified.