Kingdom Hearts: Mickey's Adventures of The Little Mermaid

Kingdom Hearts: Mickey's Adventures of The Little Mermaid is a sucessful crossover.

Plot
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, Mickey, Donald, and Goofy arrive and are reunited with Ariel, Sebastian, and Flounder; though Donald and Goofy quickly re-adapt to their sea creature forms, Mickey 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 Mickey, 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 Mickey 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. Later on, Mickey 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.

Later after that, Mickey 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 Mickey, 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, Mickey, 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 Mickey 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.

Mickey arrives, but it is too late: Ariel has signed the contract. Despite Mickey'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 Mickey quickly pulls her to the surface while Ursula laughs.

Mickey drags an unconscious Ariel to the shore, where she is confronted by Eric himself and accepted into his home. Mickey, 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 Mickey and co. don't see this happen). When not watching over Ariel, Mickey, 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, Mickey 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 Mickey 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 back to her normal form and grabs Ariel, returning to the sea with Mickey, 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 Mickey 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 Mickey, Donald, and Goofy block their path. Enraged, Ursula transforms into a giant and engages Mickey, Donald, Goofy, Ariel, and Eric in combat.

Throughout "Ursula's Revenge", Mickey, Donald, Goofy, Ariel, and Eric work together to fight Ursula, eliminating Flotsam and Jetsam in the process. In the end, Mickey knocks the trident out of Ursula's hands, and Eric impales Ursula with it, destroying 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, Ariel has become human and is on a wedding cruise with Eric, witnessed by Mickey, Donald, Goofy, King Triton, Sebastian and Flounder and the inhabitants of kingdoms. Though Ariel is sad that they have to leave again, Mickey, Donald, and Goofy refuse to say goodbye and assure their friends through a musical number that they will meet again.