Sora, Rein, Donald, and Goofy were riding in their gummi ship as they flew their way to another world. Rein had been quiet the entire ride and was starting to worry Sora, Donald, and Goofy. Sora was about to ask Rein what happened between her and Riku but stopped when their ship was being attacked by Heartless ships. They barely survived by the time they got through all the Heartless ships.
"Is it just me, or were we attacked by more Heartless than usual."
"I agree with Goofy. We have never encountered that many Heartless ships until now."
"Do you think Maleficent is onto us and is sending more Heartless?"
"Who knows." Donald noticed an underwarter world called Atlantica up ahead and steered the ship towards it. "Okay, guys. Prepare for landing."
"Land where? In the sea? We'll drown."
"Sora's right, Donald. We can't hold our breath too long."
"We can with my magic. Just leave it to me."
With Donald's magic, when they teleported from the ship and arrived in Atlantica, they were in forms that helped them adapt to being in the underwater world. Donald was half-octopus, Goofy was a turtle, Sora was shirtless and had his legs replaced with a blue dolphin tail, and Rein was now wearing a light blue tube top and now had a light blue mermaid tail. Unfortunately, their new forms didn't exactly give them to skills to swim in their new forms.
"I can't believe I'm a mermaid! I can actually swim! Kairi and I read a book about mermaids and imagined what it'd be like. I can't wait to tell her about this!" Rein tried swimming to Sora but ended up bumping into him since she didn't know how to completely stop. "Oops. Sorry Sora. Huh?" Rein looked behind Sora and noticed someone swimming towards their way. "Someone's heading this way."
Rein carefully swam around Sora and up the red-haired mermaid and yellow blue-striped fish, making sure to not to bump into them. At the same time a crab almost swam into the upside down Donald and got freaked out before quickly back to the red-haired mermaid.
"Oh relax, Sebastian. They don't look like one of them." Sora and Goofy swam towards them as the mermaid looked back at the fish who hiding behind her. "Right, Flounder?"
"I don't know. There's something weird about them." Flounder looked from behind the mermaid and towards Rein. "Maybe except her. She's a mermaid like you."
"Ah ha ha. What do you mean?"
"Hm." The mermaid swam towards and around Sora to examine his appearance. "They seem...a little different." Ariel then looked towards Rein who was making sure Donald doesn't start a fight with Sebastian. "Most of them, anyway." She then turned her attention back to Sora before stopping in front of him. "Where are you from?"
"We're from kind of...uh...far away. And we're not really used to these waters."
"Oh, I see. In that case..." The mermaid looked around until she found Sebastian with Donald. "Sebastian can show you how we swim around here. Right, Sebastian?"
"Ariel, King Triton will not like this!" Ariel told him not to worry before swimming off to leave him with the newcomers. "Easy for you to say... Okay, it's time you learn how to swim properly." He instructed on the basic before gesturing to Flounder. "Practice swimming with Flounder. Try to tag him. All right, begin."
Flounder swam off before Sora and them swam after him, tagging him each time they reach him. Once they got the hang of swimming around, they swam back to Sebastian to learn some self-defense. But before they could start the lesson, Ariel pointed out some Heartless swimming towards them. Ariel swam into a large hole while Sebastian and Flounder hid inside the nearby giant clam. Sora, Donald, and Goofy summoned their weapons and started fighting the Heartless while Rein shot her Purity Bursts. Once the all the Heartless were gone, Ariel swam out of the hole and over to the giant clam. She and Rein tried opening it, but it wouldn't budge. Sora swam over to the clam and was able to open it by hitting it with his Keyblade. Once they were out, Sebastian and Flounder swam over to Ariel.
"Those creatures chased us here."
"Oh no! These monsters might be heading for the palace, too!"
"Then we'd better head back wight away!"
"But w-what if we run into more of those monsters on the way back?"
"We'll come with you." Sora swam over to Ariel before looking back at his friends. "If we run into more, we can take care of them. Just leave it to us."
"We'd really appreciate it." Ariel then pointed at a drawing of a trident on the wall. "The trident markers on the walls point the way to the palace. We won't get lost as long as we follow them."
"Okay. Let's get going."
As they are swimming to the palace they encountered some Heartless. Sora, Donald, and Goofy fought the Heartless while Rein protected Ariel and her friends, shooting Purity Bursts at any that get too close. As they were swimming into the palace, some Heartless were swimming after them until they were destroyed by a lightning attack.
"That was too close." They looked to see that the one who shot the lightning was an older merman sitting on a throne. "As long as I have my trident, I will not tolerate those creatures inside this palace." Ariel swam up to the merman with a happy look on her face until, "Ariel! When will you listen? It's dangerous out there! Strange creatures lurk outside."
"Ahem." Sebastian has swam to the merman's side before gesturing to the merman. "Behold. You swim before the ruler of the seas: His majesty, King Triton!"
"It's nice to meet you, your majesty." Rein swam ahead of her friends before curtsying...sort of. "My friends and I have journeyed from afar."
"Who are they?"
"They helped us fight off those creatures, daddy."
"Hmm. They don't look familiar."
"We're from an ocean very far away."
"Yup. We came to find the Keyhole."
"The what?"
"What's that?"
"Ah-hyuck. Well it's a-"
"There's no such thing." King Triton interrupted Goofy before Rein could. "Certainly not here." Ariel tried to argue but stopped by seeing how serious her father looked. "You are not to leave this palace. Is that clear?"
Ariel gave her father a defiant look before swimming out of the throne room with Sora and his friends swimming after her. They followed her out of the kingdom while defeating any Heartless that appear. Rein saw a dolphin that looked scared and swam over to try and calm it, but it swam away when more Heartless showed up. Ariel led them into a cave that she calls her secret grotto. Inside they found shelves of a large variety of items.
"Look at all of the wonderful things Flounder and I have been collecting. I think it's all from the outside world." Rein swam over to a small chest and opened it to find nothing inside. "Someday, I'm going to see what's out there. I want to see different worlds. Isn't that strange?"
"No. Not at all. Rein and I used to feel the same way."
"Used to?"
"We still do." Sora looked over at Rein who got startled by a small fish that swam out of a boot after she poked it. "We want to see what other worlds were like too."
"Why don't we try looking for that Keyhole you were talking about?"
"But you father said-"
"Oh, he's always treating me like a little girl. He never wants to let me do anything. He just...He just doesn't understand."
"I'm sure he's just looking out for you." Rein swam over to Ariel to talk with her while the boys looked around the grotto. "Father's can be like that. Try talking to him."
"I've tried. But he just won't listen."
"Then try again. It won't hurt to keep trying." Seeing how said Ariel was, Rein decided to change the subject of the conversation. "You sure have a large collection. Where did you find them?"
"Huh? Oh, Flounder and I sometimes find them just lying on the ground. But we usually find most of them in old ships. Let me show some of them." Ariel grabbed Rein's hand and swam over to a fork. "This is called a dinglehopper."
"A dinglehopper?"
"Uh huh. My friend Scuttle told me its used to comb hair. See." Ariel picked up the fork and started coming her hair with it. "You should try it."
"Um...no thanks. I'm good."
"Hey Rein." Rein looked towards Sora to see him, Donald, Goofy near the exit. "Flounder told us about a big fish who might help us get passed that strong current we saw if we defeat the Heartless that're scaring him."
"I think he's talking about that dolphin we saw earlier. It was really scared of the Heartless. Let's go!"
Sora, Donald, Goofy, Flounder, and Ariel had already swam out of the grotto as Rein was about to follow when she noticed trident-shaped imprint on the wall. She was about to swim over to it until she heard Sora calling her and swam out of the grotto, not noticing the sinister-looking eels watching nearby. They found the dolphin and started defeating the Heartless while Ariel and Flounder stayed somewhere safe during the fight. After defeating all of the Heartless Sora, Rein, Donald, and Goofy rode the dolphin to an old sunken ship.
"Woah! A sunken ship!" Sora swam around the ship while his friends followed. "This is huge compared to the raft we made!"
"Ariel mentioned finding a lot of stuff in her grotto in old ships. This must be one of them." Rein saw the boys swim through an opening in the ship and quickly swam after them, not noticing the dark shadow swimming nearby. "Wait for me guys!" She found the boys near an old chest and swam towards them. "Can't believe you guys left me."
"Oh come on, Rein. There's nothing to be afraid of. Unless we run into a shark or something."
"Please don't say that, Sora. Heartless have been randomly appearing. You never know what'll show up next." Right after Rein said that a large shark broke through the wall and tried to grab them with its jaws. "Ah! Swim away!"
They all swam to avoid getting caught until the shark swam out of the ship. Sora, Donald, and Goofy swam out of the ship to fight the shark while Rein swam over to the chest. She opened it to find a crystal in the shape of a trident. She picked it up before swimming out of the ship through the large the hole the shark made. Luckily the boys had already defeated the shark by the time Ariel and Flounder caught up with them.
"Are you guys okay? We saw the shark swim out of here. We were lucky it didn't see us." Ariel saw the crystal trident in Rein's hands and swam over to get a closer look. "Did you find this in that ship? It looks like the top of daddy's trident but different. I know a perfect place to put it in my grotto."
As they swam back to Ariel's grotto the boys fought more Heartless while Rein kept Ariel and Flounder safe as well as trying not drop the crystal trident. When they made it to the grotto, Rein inserted the crystal into the trident-shaped imprint. Rein suddenly sensed something reaching out to her. But before Rein to determine what it was or where it was coming from, she got distracted when King Triton showed up.
"Ariel, you've disobeyed me again! I told you not to leave the palace!"
King Triton saw the crystal trident and became angry as he destroyed it with the power of his trident. Ariel got upset and swam out of the grotto. Rein realized she could no longer that feeling from before.
'What was that I sensed just now? Was it possible that it was this world's Heart?'
"You two, you're not from another ocean. You're from another world. Aren't you?" Sora, Rein, Donald, and Goofy were shocked that King Triton knew their secret. "Then you, young man, must be the key bearer."
"How did you know?"
"You may fool Ariel, but not me. You don't know your dorsal fin from your tail. And you, young lady, may look like one of us but I can still tell you're from another world as well. As the key bearer, you must already know...One must not meddle in the affairs of other worlds. You have violated that principle. The key bearer shatters peace and bring ruin."
"Aw, Sora's not like that."
"Goofy's right. We've been stopping the Heartless from corrupting the Keyholes of the worlds we've been on. We've also saved others from the Heartless like we saved Ariel and her friends."
"I thank you for saving my daughter." King Triton turned around and started swimming out of the grotto. "But there is no room in my ocean for you or your key."
After a while, Rein looked at Sora and saw him looking his Keyblade before swimming out of the grotto and towards the palace. She saw King Triton swimming ahead of her and swam faster to catch up with him.
"Your Highness, I understand you're thinking for the sake of your world when you tell us to leave but please listen to what I have to say." King Triton stopped swimming as well as Rein to listen to what she wants to tell him. "One of the reasons we're here is to find this world's Keyhole so Sora can use his Keyblade to lock it. If you know where it is, you can help us by te-"
"That will not be necessary."
"But the Heartless-"
"will not be able to find it. I have made sure of it. Otherwise you would have been able to sense this world's Heart." Rein was about to say something but decided not to. "Now I must get back to the palace while you and your group leave."
With that said, King Triton swam ahead while Rein started swimming back towards Ariel's grotto. When she was almost there some Heartless started appearing so she started fighting them. She sensed more Heartless behind her quickly turned around to see them get destroyed by her friends. After they have defeated all the Heartless they were discussing on whether or not to leave at that moment until Rein suddenly sensed a heart in danger.
"Something's wrong. I'm sensing a heart...no two hearts in danger! I think they're towards the palace."
"Then we better hurry!"
Sora, Rein, Donald, and Goofy quickly swam towards the palace and saw an octopus woman floating before an injured King Triton and a distressed Ariel. Before they could reach her, the octopus woman disappeared with a dark cloud of ink with the king's trident in hand. Rein quickly swam over to King Triton and hovered her hands over the king as she concentrated on using her healing ability. Her hands started glowing green for a while before it faded. Ariel helped her father sit up but the king was still too weak to get up off his throne.
"I'm sorry I couldn't completely heal you. But I was able to heal some of your injuries. It doesn't really help a lot, but even a little bit can count, right?"
"Yes...thank you. Since I'm not in the condition to do it...you must get the trident back. Ursula draws her power from her cauldron. You must hit it with magic in order to defeat her."
"You can count on us. Come on, let's go!"
Sora, Rein, Donald, and Goofy started swimming out of the throne room until Ariel swam in their way. She wanted to go with them to stop Ursula since she blames herself for helping her get the trident. With Ariel and Sebastian added to their group, the six of them started heading towards Ursula's lair with Ariel showing them the way. They swam to where they found the sunken ship and found the entrance to Ursula's lair blocked by a large rock. It also turns outs the large shark before has come back for a rematch. The Rein and the boys fought the shark while Ariel and Sebastian find a way into the lair. Sebastian found a switch and pushed it which cause the rock to lower itself and reveal the entrance to Ursula's lair. They all swam into the lair and found Ursula in the center.
"Goofy and I will keep Ursula and her eels busy. Donald and Rein, you guys focus on the cauldron."
According to his plan, Sora and Goofy fought Ursula and her eels while Donald and Rein attacked the cauldron with their magic. They noticed that with enough attacks on the cauldron, Ursula can be stunned. After a tiresome battle, they were able to defeat Ursula, forcing her to retreat with the trident. They tried going after her but the exit she took was immediately closed behind her.
"I can sense the darkness in Ursula's heart. But I think its fading the farther she goes."
"Now how are we going to go after her? We might lose track of her if we go the other way."
"Donald has a point." Sora looked around the lair exit but couldn't find any other than the way they came through. "But it looks like we don't have any other choice. But I think there was a path we didn't take yet. Let's go."
The path Sora was talking about involved getting past the strong current. But thanks to Ariel teaching them how to swim faster, they were able to swim against the current and found a path leading to the open part of the ocean. They saw Ursula and started swimming towards her.
"You pathetic fools! I rule the seas now!" Ursula was engulfed in a giant cloud of ink as she grew into gigantic version of herself. "The sea and all its spoils bow to my power!"
They started fighting Ursula which was a lot more difficult than when they previously fought her. Donald and Goofy got stunned after being hit with too many of Ursula's attacks and were being sucked in towards Ursula's mouth. Rein and Sora quickly swam over and grabbed their friends as they swam against the current pulling them towards Ursula. When the current stopped, Rein healed Donald and Goofy enough so they can help fight. As they struck Ursula with their attacks at the same time, they were able to defeat the sea witch. Ursula retreated for good this time and left the trident behind. Ariel grabbed the trident before they all started heading back to the palace. After returning the trident Ariel apologized to the king but the king took the blame instead.
"It's my fault. You followed Ursula because I wouldn't let you follow your heart. And when you found that crystal, I lost my temper and destroyed it."
"Oh, yeah, the crystal! Why did you destroy it?"
"The crystal held the power to reveal the Keyhole. The Keyhole is dangerous. I had to keep you away from it at any cost. Key bearer, I have one more request. Seal the Keyhole. My trident also holds the power to reveal the Keyhole. Will you do it?"
"Of course. That's what we had in mind from the start."
"But where is the Keyhole, daddy?"
"You should know better than anyone."
"It's in your grotto, Ariel." Everyone except King Triton were surprised that Rein already knew the location. "After we placed the crystal I sensed something reaching out to me. But that feeling went away after King Triton destroyed it. It was this world's Heart, which means that's where the Keyhole is."
"She is right. Ariel, you may borrow my trident so that you can use it to reveal the Keyhole."
King Triton handed Ariel his trident before they swam out of the throne room and to Ariel's grotto. When they got there, Ariel used the trident to reveal the Keyhole before Sora used his Keyblade to seal it.
"Sora, Rein?" Ariel asked as she swam up to them. "Your world, what's it like?"
"Oh, about that...Sorry for lying to you."
"We didn't want to tell you the truth because we're not supposed to reveal the existence of other worlds."
"It's okay. Besides, if you can travel to other worlds, may I can too. So many places I want to see. I know I'll get there someday. I'll find a way somehow. I'm sure of it." Ariel swam over to Rein and held her hands, catching Rein by surprise. "Thank you...both of you, for giving me hope that I may actually see different worlds."
"I'm sure you will, Ariel. Just follow your heart. And remember what I told you. Keep trying until you and your father have a compromise."
"I'll try. But it might not be easy."