As the Mermaid Princess Lucia undergoes a rite of passage, Gaito, the main antagonist (who resembles Kaito), and his minions crash in to kidnap Lucia and steal her pearl. Aqua Regina, the sea goddess, appears, gives Lucia a new song, and tells her that she must summon her quickly in order save the Marine World.
Tomorrow is the anniversary of the day that Lucia (in her mermaid form) and Kaito met.
Elsewhere, a beautiful emerald found in the ruins of Rina's kingdom becomes the star attraction of a cruise liner gallery, and Eriru attempts to steal it to lure the mermaid princesses to her.
Lucia sets out to make a bento for Kaito, in high hopes that it will encourage him enough to help him win his next surfing competition.
The story basically follows the main character Lucia Nanami, Mermaid Princess of the North Pacific Ocean, searches for and woos Kaito Dōmoto, a boy she rescued seven years before, while at the same time, she and Hanon Hōshō and Rina Tōin, Princesses of the South and North Atlantic Ocean respectively, battle a minion sea monsters using their voices and some magic microphones.
Meanwhile, one of Lucia's idols checks into the hotel under a secret identity.
A new sea demon, Yuri, appears during a concert given by Taro and uses her music to hypnotize the attendees and attack the mermaid princesses, who are also there.
Rina reveals herself to be the mermaid princess of the North Atlantic and then makes acquaintances with Lucia and Hanon.
The Mermaid Melody Pichi Pichi Pitch animated television series is based on the shōjo manga of the same name created by Michiko Yokote.
The episodes, produced by the animation studios Actas and Synergy Japan, are directed by Yoshitaka Fujimoto, written by Junki Takegami, and features character design by Kazuaki Makida, who based the designs on illustrations by Pink Hanamori.