In the underwater kingdom of the North Pacific, Princess Lucia of the Mermaid Kingdom is tasked with protecting the Pink Pearl, one of the seven magical pearls that maintain peace in the oceans. When the kingdom is attacked by the evil Gaito and his henchmen, who seek to steal the pearls and conquer the underwater world, Lucia flees to the surface. There, she transforms into a human and meets Kaito Domoto, a boy she once saved years ago. Lucia discovers that she can transform into a mermaid when in contact with water and uses her singing voice, enhanced by the Pink Pearl, to fight against Gaito’s forces.

As the story progresses, Lucia is joined by two other mermaid princesses: Hanon Hosho of the South Atlantic, who wields the Aqua Pearl, and Rina Toin of the North Atlantic, who possesses the Blue Pearl. Together, they form the group Mermaid Princesses and use their magical voices to combat Gaito’s minions, who aim to spread darkness and despair across the oceans. Each princess has a unique ability tied to their pearl, and their songs serve as both weapons and a means to inspire hope.

The narrative explores the challenges the princesses face as they balance their duties as protectors of the seas with their lives on land. Lucia struggles with her feelings for Kaito, who initially does not recognize her as the mermaid who saved him. Hanon and Rina also navigate their own personal conflicts, including relationships and self-doubt. The series delves into themes of friendship, love, and perseverance as the princesses work together to retrieve the stolen pearls and restore harmony to their kingdoms.

Gaito’s motivations are gradually revealed, tied to a tragic past involving his love for a mermaid princess and his desire to overthrow the existing order. His plans escalate as he gathers allies, including the mysterious Mikaru, who has a connection to Lucia. The story builds toward a climactic confrontation where the Mermaid Princesses must unite their powers to defeat Gaito and prevent the destruction of both the human and mermaid worlds. Throughout the series, the interplay between the characters’ personal growth and their battles against darkness drives the narrative forward, creating a blend of magical girl action and emotional storytelling.