Crystal show Casper all about these mermaids and one is very special and she like the world on land and specially a young prince.
Once upon a time in Deedstown, in a haunted manor, Whipstaff Manor, Casper wake up and he hear the house leak and it's a silly or funny noise and it's the Ghostly Trios cousin, Hairy Scary and Casper call his uncles that Hairy Scary has returned, because he is very silly or scatterbrained and the Ghostly Trios (Stretch, Stinkie and Fatso) kept telling him how much time does they have to tell him to stay away from Casper, he knows not to go near him because he was in the past were Yogi Bear and his friends, Boo-Boo Bear, Snagglepuss, Huckleberry Hound, Quick Draw McGraw and Augie Doggie and Doggie Daddy on Christmas time, including helping the orphan children on Halloween and in the Treasury Islands, he was using a revenge on Casper and made the Ghostly Trios not very happy to Hairy Scary and kept telling him that they are not going to speak to him again.
Hairy Scary told Casper and the Ghostly Trios that he was with a witch and her second sidekick have to trap Casper to steal all of his solds but the Ghostly Trios can't have that so they yell at Hairy Scary to get out of their home immediately then the four of them begin to fight while Casper is hearing a strange music that came from the sky so he fly up and it was Crystal playing her harp. Crystal wants Casper to help her with something so he followed her and fly down to the ocean, and they can breathe under water and they meet the little mermaid named Loofa and her starfish, Hoho, and they are playing tag with a seaweed so they have do it, but her cousin, Lita told her that her father, King Triton and her three sisters would like to see her, Hoho, Casper and Crystal immediately.
Loofa live at Merfolk Palace with her father, King Triton and her three oldest sisters, and they have a problem there's a group of garbage floating down into a sea so Loofa can take care of it with her singing voice while Crystal uses her harp to play so the ocean can be cleaned, Casper asks Crystal how did she and Loofa do that, she told him that every time the ocean gets covered by garbage, Crystal uses her harp and Loofa uses her voice to make them go away.
Above the ocean, the kingdom of Fjord, King Fjord asks his son, Prince Wilbur to be engage to another king, King Sonic's daughter, Princess Emiline but she didn't know anything else about supernatuaral creatures like ghosts, demons and witches, her father plans to murder King Fjord and Prince Wilbur so they he and Emiline can steal their money, that's will trick them to let Wilbur to marry Emiline. But King Fjord and Wilbur doesn't know about it as they set sail to Sonic's Kingdom, Wilbur's heart belongs to someone else who he haven't met yet.
Back in the ocean, Casper, Crystal, Hoho, Lita and Loofa'a three oldest sisters surprised Loofa because today it's her 16th birthday, King Triton told Loofa where's Casper and Crystal come from the surface above water and there's some creatures are looks resembles to mermaids and angels but with out tales and wings and they are called humans, so Casper and Crystal takes Loofa with Hoho up to the surface and there's a ship who is coming from the castle and it's Prince Wilbur and King Fjord.
Then suddenly there's a storm and it's striking on the ship, King Fjord and his men are are safe on the life boats but not Wilbur because they can't find him because he's drowning under water so Casper and Loofa swim to save him and bring him back to the shore and Loofa has to sing him a song until she and Casper hear somebody whose coming and it's King Fjord and his men so Casper and Loofa jump back to the ocean so they, Crystal and Hoho can see the king realizes that his son is okay and Loofa is beginning to fall in love with him and Casper, Crystal and Hoho are shock.
Meanwhile at Deedstown, the Ghostly Trios can't find Casper anywhere and it's all Hairy Scary fault because if he haven't come back, Casper would never disappear like that, so the Ghostly Trios put Casper in the toilet and flesh him away and the Ghostly Trios continue their search to find Casper somewhere around the world. After that they find Casper to search around the world, they see King Sonic who's is making a revenge on King Fjord, to do that his daughter, Emiline can be engaged to Wilbur but he has a feeling that he see a mermaid who saved his life.
The Ghostly Trios can't find Casper around the kingdom of Sonic but they found the Double Angels (Ruby and Emerald) who told them that was Crystal needed their nephew help to go down the ocean and help a little mermaid. But the Ghostly Trios thought that they didn't believe in mermaids and Hairy Scary (who got out of the sewers) is being listed so he joined King Sonic and Emiline to get rid of King Fjord and Wilbur.
Back in the ocean, the King Triton told Casper, Crystal, Loofa and Hoho that they have to see the sea witch and she'll make Loofa into a human so they went to the sea witch's lare and Loofa asks her that she wants to be human. The Sea Witch can make a deal to Loofa, as soon Prince Wilbur love her, she'll remain human forever but if he loves someone else, Loofa will turn into a sea foam (just like her aunt, Marina). Loofa will do it she'll take a potion if she can trade her voice to the sea witch and she did, so she, Casper, Crystal and Hoho returned to the surface immediately.
As they get up there, Loofa drinks the potion and her tale turn into legs and she fell unconscious, Casper and Crystal started vanished because Wilbur came and he take her to home see everyone. Meanwhile in the Anti Kingdom, Emiline, King Sonic and Hairy Scary start to build a mega robot-like vehicul/machine that it resembles to a monster from national mythologies and it can cause King Fjord and Wilbur on his and Emiline's wedding day.
In Wilbur's bedroom, Loofa wakes and she's in the prince bedroom and she than realizes that she has legs and Casper and Crystal with Hoho came and see Loofa is now human but she can't talk without her voice. They hear Wilbur is coming in so Casper and Crystal begin to vanish again, they have a plan to get Loofa's voice back but they don't know how yet but they are listen to Wilbur told Loofa that he's engaged to King Sonic's daughter, Emiline but his heart belongs to a mermaid who saved his life and he doesn't know what mermaid, she was and Loofa can't say anything because she lost her voice.
Later Casper and Crystal help Loofa to learn how to walk with her legs and they smell something delicious down in the dining room so they went down there and see some food for the wedding but they hear someone else coming and it's King Sonic and Emiline so Casper, Crystal and Loofa begin to hide under the table, King Sonic told his daughter about their revenge on King Fjord and Wilbur will lose Loofa. But Casper and Crystal aks Loofa how to stop them and the Sea Witch and Loofa will get her voice back but she doesn't know how so she sit outside and cry but Hairy Scary show up and begin to chase Casper and Crystal.
Casper and Crystal start hiding so Hairy Scary can't find them, moment later Casper and Crystal see Lita who feel sorry for Loofa. Lita told Casper, Crystal, Hoho and Loofa that she'll go to the sea witch's lare and try to get Loofa's voice and they agree with that, so Lita went to see the sea witch and asks her that she wants her cousin voice backs and the sea witch will give her cousin voice if she can give her hair to her so Lita cut her hair and give to the sea witch and she gives Loofa's voice to Lita and then she went.
Meanwhile the wedding is about to begin for Wilbur and Emiline while Hairy Scary has his robot monster to revenge on the king and the prince, but outside the wedding, Lita came back to Casper, Crystal, Hoho and Loofa and she gives Loofa her voice back and she can speak again so she, Casper and Crystal went to the church and stop the wedding and Loofa told King Fjord that King Sonic plans to murder him and Wilbur so he and his daughter can steal all of their money. King Sonic said that the plan will completed and then Wilbur and Fjord will no longer be. Suddenly, he and Emiline heard somethings and Crystal used her harp to signals for King Triton and he shows up so Sonic, Emiline and Hairy Scary start running off.
Casper and Crystal told Loofa and Wibur to make up while they chase those villains so Wilbur and Loofa kiss. As Casper and Crystal chasing Hairy, Sonic and Emiline, Crystal played her to signal her aunts as they and Casper's uncles arrived and then they came to