Glan is an orphaned young boy living in a coastal port town. Initially timid and prone to crying when frightened or overwhelmed, he possesses an inherent sense of justice and loyalty. His life changes dramatically when he encounters a mouse-like character named Jim, who thrusts a torn treasure map fragment into Glan's hands just before being captured by pirates. This event propels Glan into an unexpected adventure.

Determined to help Jim and lured by the promise of buried treasure, Glan stows away aboard Captain Silver's ship, disguised as a cabin boy. Onboard, he faces numerous dangers from the treacherous crew. Despite his initial fear, Glan gradually shows growing courage and resourcefulness. He forms a crucial bond with Katharine, a cat-like girl also entangled in the pirate plot, and their friendship becomes a vital source of support.

Glan actively participates in the perilous search for Captain Flint's legendary treasure on the titular island, contributing to solving the map's clues and navigating its traps. Throughout the journey, he confronts the ruthlessness of the pirates, particularly Captain Silver, learning harsh realities about greed and betrayal. These experiences accelerate his maturation, transforming him from a frightened orphan into a more determined and resilient individual.

His bravery is repeatedly tested, especially when protecting his friends. Glan plays a pivotal role in the final confrontation over the treasure, directly opposing Captain Silver and his crew. His actions help thwart the pirates' ambitions. Following the adventure, Glan returns to his hometown, no longer the timid boy he once was. His experiences have instilled greater confidence and a sense of accomplishment, marking a clear development from his starting point.