Elina is getting used to the new wings she received after helping save Fairytopia from the evil enchantress Laverna. While flying around she encounters a young sea-butterfly fairy who tells her that Nalu, the merman prince who befriended her on her previous quest, was recently abducted at the Crystal Cover by a number of Laverna’s minions, the Fungi. Laverna, who has been banished from Fairytopia, is now hunting for the Immunity Berry, which can bestow anyone who eats it immunity to magic. Nalu apparently can find the berry, so Laverna has had her minions kidnap him so she can get her hands on it.
If Nalu does not take them to the Immunity Berry, Fungus Maximus, their leader, threatens to employ a magical poison, which if poured into the ocean will destroy all of Mermaidia. Nalu has no choice but to do his bidding. The Fungi then leave with him and swim out into the ocean. A short time later, Elina reaches the Crystal Cove, with her puffball friend Bibble. Nalu is gone, but she does meet up with Nori, a mermaid. Nori had intended to visit Nalu, and doesn’t know he’s been kidnapped. When Elina tells her what she heard from the sea butterfly fair, Nori decides to go and save prince on her own. Elina finds some of the magic seaweed Naru gave her last time to breathe underwater and follows. In the meantime Nalu has tricked the Fungi into an underwater trap, the Carousel of Confusion, in order to delay them. Since this place has a strange effect on memory, he is able to lead the group in circles, but eventually Fungus Maximus figures how to get out of it.
Elina get to Mermaidia and discovers Nori trying to find out to locate the oracle Delphine so that she can find out how to help Nalu. Where Delphine is located, however, is a secret only the merfairies know, and they are almost impossible to communicate with. Elina, however is able to help in the discussion and soon they get the information they need to find the oracle. From Delphine they learn they will have to swim to the very bottom of the Depths of Despair to locate the Mirror of the Mist, and with that they can find Nalu. She also informs Elina that in order to get there she will have to replace her wings in a mermaid’s tail or she won’t be strong enough and offers her a magic pearl necklace to cause the transformation. The necklace comes with a risk, however. Delphine warns Elina that she only has a limited amount of time to get the mirror. If she stays to long in the water as a mermaid, the change will become permanent.
Elina and Nori and Bible swim out to the Depths of Despair, but Nori quickly finds herself caught by a hostile sea plant. Not being able to swim strongly enough to help, Elina decides to use the necklace and turns into a mermaid. She helps Nori escape and the two swim down to the bottom of the depths and uncover the Mirror of the Mist in a cavern there. In the mirror they can see Nalu. The mirror then creates a magical bubble beacon that will help them locate where he is. Also, now on both of Nori’s and Elina’s arms appears the Crest of Courage, which they receive for surviving the swim down into the Depths of Despair. The bubble carries the three them to a cave packed with boiling hot thermal vents. Nori leads them though that dangerous cave into a cavern overflowing with magical fruits, including one that exposes the eater’s true self. Finally, the bubble takes them to the ocean surface, where they find two Fungi guarding Prince Nalu. While Bibble turns their attention away from their prisoner, Elina and Nori free him and they escape back into the water.
Nalu is appreciative of what the mermaids have done, but the Fungi have already found the where Immunity Berry is hidden. The three head to the cave and end up in a fight with the Fungi to reclaim the berry Fungus Maximus has found. They manage to secure the berry by tricking the Fungi leader with a fake berry, but the battle takes to long and Elina discovers she has become a mermaid permanently. Nori suggests she might be able to become a fairy again by eating the true-self berries they found in the cavern of magical fruit. It works; in fact the wings that she gets back are even more magnificent than what she’d gained from the rainbow necklace. Nori and Nalu confess their love for one another. Afterwards, Elina and Bibble head back to Fairytopia.
Far away, Laverna receives the fake Immunity Berry. She takes a bit out of it, but as it turned out, it was a true-self berry instead and it turns her into a toad.
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