美人鱼爱丽儿和王子埃里克结婚后过着幸福美满的生活,不久,两人的女儿麦乐迪也出生了。小家伙天性调皮,和妈妈正好相反的是,她一直向往着海底绚丽多彩的世界,爱丽儿也一直没有告诉她自己的身世。 上一集企图加害爱丽儿的巫婆的妹妹莫嘎纳为了帮姐姐报仇,骗麦乐迪说能帮她变成美人鱼,乐不可支的麦乐迪就这样跟着莫嘎纳前往了海底。丢失了爱女的爱丽儿和埃里克异常焦急,为了寻找爱女下落,爱丽儿决定重回大海。After having found a magic locket which shows the kingdom of Atlantica, Melody decides to run away from home and find the truth behind it. Ariel eventually discovers that her daughter has run away after being told by Sebastian. Ariel must turn back into a mermaid to go into the sea once more to find her missing daughter. However, the crazy sister of Ursula, named Morgana intends to take control over the entire ocean. Ariel and her friends must stop Morgana from accomplishing the mission that Ursula failed and save Melody from her evil clutches.