Prince Justin, the human prince of Gandor, serves as the story's central romantic interest. He survives a near-drowning during a shipwreck attack by a three-headed sea serpent and a giant Cyclops, rescued by the mermaid princess Marina. Mistakenly believing Princess Cecily saved him, Justin becomes engaged to her. Upon discovering Marina's true identity and form as his rescuer, he develops a close bond with her.

Seeking to overcome the barrier between their worlds, Justin receives a magic potion from his tutor, the wizard Anselm. This potion grants him temporary underwater breathing, allowing him to visit Marina's realm and navigate both land and sea. Their quest focuses on locating a legendary golden scroll or tablet capable of permanently transforming merpeople into humans, aiming to reunite their worlds.

Justin demonstrates bravery and compassion throughout his journey. He confronts adversaries including the sea witch Hedwig, who seeks to capture him and Marina to obtain the Amulet of Power for world domination, and his rival, Prince Lothar of Brakston, who cooperates with Hedwig. Allies supporting him include Anselm; his loyal page, Chauncey; and underwater creatures like Ridley the sea otter. His parents, King Charles and Queen Catherine, rule Gandor, with Queen Catherine showing particular interest in his romantic life.

Justin's development centers on his unwavering commitment to Marina and his dedication to bridging human and merfolk societies, consistently acting with a focus on justice and unity.