Almost completely underwater, Atlantica is a rocky world with immense holes on its surface. Beneath the surface is an entire world full of fish, merfolk, and other aquatic species that make up King Triton's underwater kingdom. The Heartless that appear in Atlantica are unique to it, such as Sea Neons which resemble jellyfish. In all games, the boss of Atlantica is Ursula (occasionally accompanied by Flotsam and Jetsam).
King Triton is the ruler of this world, and its Keyhole is hidden in Ariel's grotto.
Sora, Donald, and Goofy arrive on this strange new world after being given new magical forms by Donald that allow them to survive and adapt; Sora becomes a merman with a dolphin tail, Donald becomes a alf-duck squid, and Goofy becomes a sea turtle. They quickly meet up with Ariel, Flounder, and Sebastian, and Sora quickly disguises the fact that they are not from this world by claiming that they are from a distant ocean. After teaching them to swim, the group is attacked by a group of Heartless, which Sora and his gang promptly dispatch.
A concerned Ariel invites the group to visit her father's palace to see what to do. Taking them there, King Triton dispatches more Heartless with his magic trident, dismisses Sora's questions about the world's Keyhole, and grounds Ariel. Sulking, Ariel sneaks off to her grotto, where she points out a mysterious mark on the wall. Ariel decides to help them find the Keyhole against her father's wishes. The party travels to the Sunken Ship, where they find a shining Crystal Trident that looks as if it might fit in the mark.
After escaping Glut, a Great White Shark with a huge appetite, they take it back to the Grotto and place it on the wall, but before it can fulfill its purpose, Triton bursts in and destroys it. Ariel storms off, allowing Triton the opportunity to explain that he knows Sora and the rest are Keyblade bearers from another world. After proclaiming the Keyblade only brings destruction, he informs them they have no further business in Atlantica.
Meanwhile, Ariel is approached by the evil sea witch Ursula, who promises the mermaid princess that she can help her get to "other worlds", but in order to do so, she needs Triton's trident; she also reveals during this time that Sora, Donald, and Goofy are from another world. Ariel steals it and gives it to Ursula, who uses it to injure Triton before retreating to her lair. Sora, Donald, and Goofy go in pursuit, and Ariel accompanies them, wanting to atone for her mistake. After a harsh battle that ultimately ends Flotsam and Jetsam, Ursula escapes from her lair.
They relocate the witch in another location and a harsh battle commences. During this time, Sora is given the ability to swim at high speed, which allows him to evade Ursula's attacks. In the end, Ursula receives a swift kick in the tentacles from Sora and his gang; they return the trident to Triton. On the King's request (as well as their own), he and Ariel open up the world's Keyhole located in Ariel's grotto, which Sora promptly seals.
Preceding this, Ariel asks Sora what his world is like, and Sora takes this opportunity to apologize for lying to her. Ariel forgives him with a smile, confident that she can find a way to travel to other worlds just as Sora had.
Chain of Memories
Sora, Donald, and Goofy arrive in Atlantica based on their memories. They retain their original forms, but are able to breathe underwater. They run into Ariel who has stolen her father's trident to save her friend Flounder, who has been captured by Ursula to exchange for the trident.
They help Ariel rescue Flounder and eventually meet Ursula in a huge battle. After they defeat the witch, Ariel takes it back to the palace, deciding to tell her father the truth.
Riku arrives afterwards, but meets no one other than the sea witch and a battle ensues between them until Riku wins.
Kingdom Hearts II
In Kingdom Hearts II, the plot follows much to the original film. Like in the original film, Ariel is swimming on the surface shortly after a storm, and is horrified to see a young man, Prince Eric, floating on the remains of a ship. She brings him to the shore and falls in love with him, and, after singing a love song to him, returns to the ocean just as he regains consciousness.
Sometime later, Sora, Donald, and Goofy arrive and are reunited with Ariel, Sebastian, and Flounder; though Donald and Goofy quickly re-adapt to their sea creature forms, Sora has difficulty due to his year spent in suspended animation, so Ariel and co. help him. Sebastian then reveals that Ariel has been very depressed lately, and asks Sora, Donald, and Goofy to participate in the annual musical, hoping that their presence might get Ariel to lighten up. They agree, and perform "Swim This Way" fantastically, and once again meet King Triton, who tells Sora in private that perhaps his participation in the musical might takes Ariel's mind off her fascination with the human world.
However, things do not go as planned; even with Sora's presence, Ariel is unable to focus on rehearsals. Flounder then takes the trio to a statue of Prince Eric that he found, which is sure to cheer Ariel up. After acquiring the Magnet spell, Sora retrieves the statue and places it in Ariel's grotto, surprising her with it. Ariel then expresses her desire to walk on land in "Part of Your World", and Sora promises to help her in any way that he can. Sebastian is distressed by this secrecy.
After increasing the power of his Drive Gauge, Sora and Ariel perform spectacularly in "Under the Sea", a musical number which Sebastian hoped would "cure" Ariel's obsession with life on land. However, Ariel would prefer to sing about how the creatures of land and sea can live together in harmony, and ultimately decides not to sing at all when Sebastian shoots that possibility down, storming off with Sora, Donald, and Goofy in tow. Sebastian proceeds to complain about Ariel falling in love with a human at "the worst possible time"; unfortunately, Triton overhears him, and is not happy, especially given the fact that humans were responsible for his wife's demise.
Triton confronts Ariel, Sora, Donald, and Goofy in Ariel's grotto, angry with Ariel falling in love with a human. Ariel refuses to believe that all humans are as evil as Triton says, and Triton finally loses his temper; determined to turn Ariel over to his views of humans by any means necessary, he promptly destroys the statue that Sora retrieved for Ariel with a burst of energy from his trident and storms off. Distraught, Ariel runs off.
While moping in the courtyard, Ariel is confronted by Ursula, who has been resurrected by the powers of darkness. Ursula strikes up a deal with Ariel; in exchange for her voice, Ursula will change Ariel into a human being for three days. If Ariel can get Eric to kiss her within those three days, then the spell will be permanent; if she fails, she will return to mermaid form, and her very soul will belong to Ursula for all eternity. Sebastian and Flounder witness this, but are prevented from going for help by Flotsam and Jetsam.
Sora arrives, but it is too late: Ariel has signed the contract. Despite Sora's best efforts to stop her, Ursula magically draws Ariel's voice into her shell necklace and transforms the mermaid princess into a human. Ariel, no longer able to breathe underwater, begins to drown, but Sora quickly pulls her to the surface while Ursula laughs.
Sora drags an unconscious Ariel to the shore, where she is confronted by Eric himself and accepted into his home. Sora, Sebastian, and the rest unanimously agree to keep Triton in the dark about Ariel's situation, and spend the time watching Ariel's progress; at one point, Ariel and Eric nearly kiss, but are stopped by Flotsam and Jetsam rocking their boat (though Sora and co. don't see this happen). When not watching over Ariel, Sora, Donald, and Goofy also spend the three days looking for Ursula in the hopes that they can force her to reverse the spell, but are unable to find her.
Near the end of the final day, Sora and co. check up on Ariel, and see her crying at the beach. At that moment, Eric, having seemingly been hypnotized, walks to the beach alongside a black-haired girl named Vanessa singing in Ariel's voice. Seeing the girl wearing Ursula's necklace, an enraged Sora puts two and two together and uses a burst of energy from the Keyblade to destroy the necklace, breaking Ursula's hold over Eric and restoring Ariel's voice. Eric realizes that Ariel is the girl who saved him, but before they can kiss, the sun sets; Ariel has missed the deadline. Ursula reverts to her normal form and grabs Ariel, returning to the sea with Sora, Donald, and Goofy in hot pursuit.
In the confrontation that follows, Triton arrives and attacks Ursula, but she reveals that even he doesn't have the power to break Ariel's contract. With no other choice, Triton agrees to take Ariel's place, and is transformed into a polyp as the horrified Sora and co. watch. Ursula promptly takes Triton's trident and proclaims herself the queen of the sea, moving to finish Triton off.
Ariel, restrained by Flotsam and Jetsam, demands that Ursula let him go, but at that moment, Eric arrives and hits Ursula in the arm with a harpoon. Flotsam and Jetsam charge at Eric, but Sora, Donald, and Goofy block their path. Enraged, Ursula once again transforms into a giant and engages Sora, Donald, Goofy, Ariel, and Eric in combat.
Throughout "Ursula's Revenge", Sora, Donald, Goofy, Ariel, and Eric work together to fight Ursula, once again eliminating Flotsam and Jetsam in the process. In the end, Sora knocks the trident out of Ursula's hands, and Eric impales Ursula with it, defeating her once again.
In the aftermath, Ariel reveals her mermaid form to Eric, and sadly states that they cannot be together. Eric, however, states otherwise, and both embrace. In the end, King Triton accepts his daughter's choice and Sebastian goes along with Ariel's idea for the musical through "A New Day is Dawning", in which Ariel proclaims her newfound happiness.
After the success of the music, and Sora uses the Keyblade to open another pathway. Though Ariel is sad that they have to leave again, Sora, Donald, and Goofy refuse to say goodbye and assure their friends through a musical number that they will meet again.
In the ending credits, Ariel has become human and is on a wedding cruise with Eric, witnessed by King Triton, Sebastian and Flounder and the inhabitants of kingdoms.
Atlantica has been the most drastically changing world throughout the Kingdom Hearts games. In each game it appears in, its gameplay is completely different. In Kingdom Hearts, Ariel joins Sora as a party member and things carry out much like in any other world, with the exception that the controls are different to fit Sora's new form. You are also restricted from using special attacks such as Ars Arcanum. In Kingdom Hearts: Chain of Memories, Sora doesn't change form at all, and the world is treated just like all other worlds. In Kingdom Hearts II, possibly the biggest change is seen; only two areas are able to be traveled and the story is not progressed by battling enemies, but by singing.
Kingdom Hearts gives Atlantica a sprawling design that requires certain special abilities to navigate successfully. Sora and company arrive in the Tranquil Grotto, where Ariel and Flounder host the swimming tutorial. From there, the party travels forward, practicing battle in the Undersea Valley, following the trident marks to their final destination. One exit leads to the ironically-named Calm Depths, which plays host to a swift moving current that pushes Sora and his party around unless they use Ariel's dolphin friend or obtain the Mermaid Kick ability.
The Mermaid Kick ability replaces the Dolphin originally required to get through the calm depths. Following the trident mark here takes the group to the Undersea Cave with a strong current of its own. Making it past the current leads to the Undersea Gorge, a vast area which leads to several other places in Atlantica. On the opposite side of the gorge are the entrances to Ariel's Grotto and, under the crate, the end of a hidden tunnel that leads to the Sunken Ship. Somewhere in the middle is the entrance to Triton's Palace and the Throne Room within. Also there, but easily missed, is the Undersea Garden, hidden to the left of the Palace entrance.
After getting into the Sunken Ship area, Sebastian helps the party find a hidden door in the corner which leads to the creepy Den of Tides; the right fork leads to the Cavern Nook, a room with a save point, while the left fork takes the party to the Tidal Abyss and, past that, Ursula's Lair for the first of two encounters with the sea witch. Finally, using Mermaid Kick from the Calm Depths, Sora and the party can reach the very end and find the entrance to the Open Ocean, where their final encounter with Ursula takes place.
Kingdom Hearts II features a tiny Atlantica in comparison; only two areas are available for Sora and the party to freely explore. They arrive and interact the most in the Undersea Courtyard, where the song mini-games are activated. On the opposite end from the save point is the entrance to Triton's Throne, which holds other characters, but no Heartless. Ariel's Grotto, The Shore, and the Wedding Ship also appear, but only in cutscenes.
Atlantica is unique in that Sora takes on a different form while on the world's "surface", courtesy of Donald Duck. While in Atlantica, he is transformed into a "dolphin merboy", while Donald himself becomes a half-duck half-octopus creature, and Goofy a sea turtle.
It should be noted that this change of appearance does not take place in Chain of Memories or Re:Chain of Memories, and that while all the other residents of Atlantica (notably Ariel, Flounder and Sebastian) swim around, Sora is still seen running around in his red shorts. This is because they are not actually underwater, but in a series of White Rooms in Castle Oblivion.