After the final battle against Gaito, Sara had given her life to remain with him. The sea world is now at peace, but only with six mermaid princesses. While Caren, Noel, and Coco have returned to their kingdoms, it should be time for Lucia, Hanon, and Rina to do the same. After all, Mitsuki Tarou is leaving for Germany, and Doumoto Kaito is heading to Hawaii. Lucia is devestated, and when things couldn't get any worse, Kaito is lost at sea! Worst of all, a strange angel named Michel has appeared, wanting the mermaids to join him. But on the lighter side, Sara's spirit tells Lucia of Sara's successor, a young mermaid named Seira. The mermaids are in for another adventure!

As the mermaid princess of the North Pacific (one of the seven mermaid kingdoms), Lucia entrusts a magical pearl to a boy who falls overboard a ship one night. Lucia must travel to the human world to reclaim her pearl and protect the mermaid kingdoms. Using the power of music Lucia is able to protect herself and the mermaid kingdoms from a growing evil force.

Ichigo Momomiya expected her date with her crush, Masaya Aoyama, to have a more romantic ending than her falling asleep after she sees a vision of a cat entering her body. Things get even stranger when, the next day, she begins displaying cat-like behaviors such as an intense craving for fish. When Masaya is attacked by a monster, she discovers that she is able to transform into the cat-eared magical girl, Mew Ichigo. After she saves Masaya, two researchers named Ryou Shirogane and Keiichiro Akasaka approach her and reveal they had injected her with Iriomote Cat DNA in order to create a magical girl that can save the world from aliens. Ichigo is tasked with fighting these monsters, but saving the world proves to be just too much for Ichigo to handle by herself. Fortunately, she learns that there are four other girls just like her, and now, Ichigo must find them quickly before the aliens completely take over the planet.