Following the epic clash with Gaito, Sara sacrifices herself to be with him in the sea world, leaving behind a tranquil realm with only six mermaid princesses. While Caren, Noel, and Coco have returned to their kingdoms, it's time for Lucia, Hanon, and Rina to bid farewell as well. With Mitsuki Tarou off to Germany and Doumoto Kaito bound for Hawaii, Lucia is heartbroken, especially when Kaito vanishes at sea. To add to the turmoil, a mysterious angel named Michel surfaces, urging the mermaids to join him. However, amidst the chaos, Sara's spirit reveals the existence of her successor, the young mermaid Seira. Brace yourselves for another thrilling escapade as the mermaids embark on a new journey!