In Search of the Castaways tells the story of a rescue expedition launched after a message in a bottle is discovered inside a captured shark. The damaged document reveals that Captain Harry Grant has been shipwrecked and is stranded somewhere along the 37th parallel south.

Determined to save him, Lord and Lady Glenarvan organize a voyage accompanied by Grant's children, Mary and Robert. Their journey takes them across South America, Australia, New Zealand, and remote regions of the Southern Hemisphere as they follow the few clues contained in the mysterious message.

Along the way, the travelers face natural hazards, hostile environments, unexpected setbacks, and dangerous adversaries. Despite numerous difficulties, they remain united by hope, courage, and perseverance. The expedition is aided—and occasionally complicated—by the absent-minded geographer Jacques Paganel, one of Jules Verne's most memorable characters.

Combining adventure, exploration, geography, and scientific curiosity, the novel celebrates friendship, determination, and the spirit of discovery. Considered one of Verne's finest adventure novels, it is the first installment of the trilogy that later continues with Twenty Thousand Leagues Under the Seas and concludes with The Mysterious Island.