Sofia and her family are visiting Merroway Cove. King Roland tries to go fishing, but has some issues with the fishing rod, while Sofia wants to meet Oona. Her father tells her to be back when the water show that Queen Emmaline prepared starts in the evening, to which Sofia promises. Sofia turns into a mermaid herself and dives into the water to search for her mermaid friend. She finds her and the others practicing for the water show. While everyone shows great tricks, Oona is only allowed to use her magical comb to conjure up a few little waves. Sofia greets everyone and comments on the fountain Cora made. Meanwhile, Oona is disappointed because her waves were barely noticeable. She tries to persuade her mother into teaching her more skills, but Emmaline denies her request, stating she is still too young for such a thing and will teach her when she is old enough. After Cora tells her sister the fountain she will make will be the star of the show, Oona gets angry and swims away, expressing how everyone treats her like a little kid unable to handle such a magical artifact. So, she tries to create a small sea twister. Oona succeeds, but the twister gets too big and is soon out of her control and it swirls away. Fearing that her mom will be furious if she finds out about her careless mistake, Oona follows the twister.
Meanwhile, Sofia has decided to search for her friend. She swims inside a cave, but gets startled by a Scare-Acuda. Another protector, a mermaid named Nerissa, appears and catches the fish. Nerissa recognizes Sofia and tells her she trained together with Chrysta. Nerissa sends the Scare-Acuda home to the north end of the cove. Nerissa explains to Sofia the reason why she is in Merroway Cove: To find Ursula's Necklace, one of the Wicked Nine which is hidden in the Kelp Forest. Nerissa leaves, heading to the hiding place of the Wicked Nine item she is searching for. Oona, still searching for her missing sea twister, swims into Sofia and asks her if she had seen it. Sofia notices something is wrong and Oona confesses her mistake. Sofia suggests to tell Queen Emmaline, but Oona tells her not to worry because she is afraid of the consequences she might face if her mother finds out. So, she pleads Sofia to help her find the sea twister. Now, Sven arrives, wanting to inform Oona about the sea twister which has gone wild, but Oona tells him she already knows about it because it is her fault. The seahorse makes the same suggestion as Sofia did earlier, but Oona dismisses him again, saying she is able to stop it herself.
Sofia, Oona and Sven swim towards the Kelp Forest, where the sea twister was last seen, only to discover the twister has moved on. While following the path of destruction the twister left behind, they help some sea creatures in need, one of them being an octopus named Octavio who was saved by Nerissa, but has seen her being dragged away by the rampaging sea twister with her Enchantlet left behind. Octavio leads them to the Stormy Shipwreck where the twister went. The group finds Nerissa trapped underneath the fallen mast of the sunken ship. Sofia frees her by binding the Enchantlets together as only one of them is not strong enough. After Nerissa is freed, they continue their search for the sea twister. Nerissa also tells them that Ursula's necklace has been carried away by the twister. When they discover the twister, Oona follows it very closely. Sven notices the chest which contains the necklace in the eye of the twister. Now, Oona tries to get the sea twister to go away, but her yelling "Go!" with the mermaid comb in her hands only leads to the twister growing in size until she cannot swim against the current any longer. Nerissa saves her, but Sofia and Sven are pulled into the twister. While the princess manages to get the necklace out of the chest, she is unable to free herself from the twister. Nerissa pulls her out again by binding the Enchantlets together again. After Sofia is free, they notice that the twister is moving towards the palace. Oona finally agrees to get her mother.
While Oona swims as fast as she can to get her mother, Nerissa and Sofia attempt to slow down the twister with some rocks. Oona arrives at the palace, but cannot find her mother, only her sister Cora. Sven tells Cora what happened. At first, Cora is angry at Oona, who tells her how she just wanted to impress the others at the show. She is sorry about that, but soon suggests using the trident to stop the twister without help from Queen Emmaline to cover up Oona's deed, even though she has only seen her mother use it. Sven is worried about that Cora cannot control the trident and leaves. Oona and Cora swim towards the sea twister with the trident but cannot stop it as every shot they fire at it misses. Sofia and Nerissa's attempt to stop it with some heavy rocks has also failed. Suddenly, Queen Emmaline arrives and stops the rampaging sea twister with the trident. As it turns out, Sven got her to help. Nerissa thanks Sofia for getting the necklace and leaves to take it to the Mystic Isles. Emmaline, on the other hand, is angry with her daughters for using the trident and the comb without permission and confines them to the Palace. But because they apologized and told the truth in the end, she allows them to take part in the show after all and announces the start of the confinement for tomorrow. Now it is time for the show, Sofia arrives in time and they all enjoy the show together. As it turns out, Roland only caught a long nap while he was fishing.
- Ariel Winter as Sofia
- Travis Willingham as King Roland II
- Kiernan Shipka as Oona
- Jodi Benson as Queen Emmaline
- Sarah Mitchell as Cora
- John Ross Bowie as Sven
- Alyssa Rhoney as Nerissa
- Hudson Yang as Octavio
- Chris Grace as Scare-Acuda
- Moral: Do not overestimate yourself and always get help if a problem gets too big.
- The protectors get Ursula's necklace.
- Sofia has a new mermaid design.