Casper can find the mysterious of the ghostly human girl and figure that they're related.
Once upon a time there was a half ghost and a half human and it's called "The Ghostly Human Girl", and her named is Jade and she was trained to scare and picked on fleshies. But her allies came to help her to scare everyone in Deedstown. But Casper and his best friend, Kat Harvey came and see those ghosts humans scared people, because Jade is an halfa human-ghost to do something bad to them. When Casper begin to fight with Jade, he asks her if he seems her somewhere before, she looks familiar to him and Jade told him the samething but not as in spirits so she pushed him down. Jade and her gang have to return to their master in the lair immediately.
Casper and Kat have to return to their home, Whipstaff manor to tell her father, Dr. James Harvey and his uncles, Fatso, Stinkie and Stretch about the ghostly human girl, but Mia Risley and her adopted father came and tell them about the ghostly human girl and she is Casper's sister. But Casper didn't knows that he had a sister. Mia told him that before one of them became a ghost Casper's parents are divorced so Jade lived with her mother and Casper lived with his dad but not anymore because he is now a ghost and lived with his uncles. But she and Casper were orphans now so Casper and Kat need help from Spooky the Tuff Little Ghost and take them to see Jade immediately.
Meanwhile, Jade and her gang made their way to see their master, Sigmondo Alvero who's very angry because a regular ghost who's not scary has stopped Jade from causing Deedstown, but Casper told him that Jade cannot scare everyone in Deedstown because she's his sister. Jade didn't realize that she had a long lost brother who is now a ghost after he lost his father just she lost her mother, and Kat replied her and Casper that they are both have some long lost siblings after their parents are deforce and never seen each other again.
When the gang have to take Jade back to Whipstaff manor, Sigmondo was very angry because his assistant is related to the regular ghost so he's decided to get rid of Casper, with him out of the way Sigmondo can have Jade back. Suddenly, there is a magic mirror and it's telling him that the young lady has been plotting to get rid of monsters and ghosts at Scare School by her evil magic but Kibosh save everyone and vanished her into the mystic abyss, that gives Sigmondo an idea so he then have a plan to destroy a friendly ghost.
Later Casper, Kat and Spooky show Jade how she remembers everything at Whipstaff manor but it seems like a strange place from her dream and she saw her mother's dress (which Kat wear for the party) and also see Casper's sledboard and he told Jade everything how did he become a ghost during a sled ride all day and got sick and his father got very sad. Jade told Casper that she lost her mother too after she became a ghostly human girl, it was all started when she was a normal human or a mortal girl her mother gave her a surfboard for her birthday and she was surfing all day but her mother told her that's enough but she couldn't stop it while she was having so much fun, then it got very dark, Jade got very sick and her mother got so sad.
Casper and Kat feel sorry about Jade, so they decided to get her to find a way to get her family back but Jade told them that she have Casper as her brother when they are alive, Casper told her that he has three uncles if she would like to meet them, and Jade would love too, so they went downstairs and see her brother's uncles, the Ghostly Trios and Kat's father, Dr. James Harvey, they told them that they found Casper's long lost sister and she was a normal child when she was surfing all day and then it got late at night and she got sick and her mother got sad, the Ghostly Trios feel so sad that Jade has a same problem as their nephew does, when suddenly there's someone coming and it's Sigmondo and he's very bossy to Jade.
Jade told Sigmondo that she's couldn't explain but he had enough of her being with regulars ghosts, and he going to get rid of a ghost, Jade met first. Casper asks Sigmondo why is he doing a thing like that to him, Sigmondo told him that all ghostly humans work together to against those regular ghosts and he has helping with some friends, Casper told Spooky and the Ghostly Trios to fight Sigmondo and his gang while he and Jade protected Kat and Dr. Harvey somewhere and the fight is on, but it was a bit that Sigmondo and his gang captured Spooky and the Ghostly Trio, now they need left are Jade, Casper, Kat and Dr. Harvey.
Casper, Kat, Dr. Harvey and Jade feel sorry for themselves and Jade is the only one who's feeling sorry for her and she has never become a ghostly-human girl, she should have become a regular ghost like Casper, Spooky and the Ghostly Trios. She told Casper, Kat and Dr. Harvey that they all need help to stop Sigmondo and save Spooky and the Ghostly Trios, but Casper, Kat and Dr. Harvey told her that they need help to stop the evil Sigumndo and his henchmen but Mia and her fadoptive ather came with Wendy, Peter, Miranda and the Witch Sisters (Gerti, Gabby and Fanny) and decided to help Jade, Casper, Kat and Dr. Harvey to rescue Spooky and the Ghostly Trios.
Meanwhile in Sigmondo's lair, he and his henchmen trapped Spooky and the Ghostly Trios in the force field and they demanded them to let them go but Sigmondo can't and he wants now are Jade, Casper, Kat and Dr. Harvey. He and his henchmen plans to take over the world using an hypnotic machine, but suddenly Jade, Casper and the gang came and demanded Sigmondo and his gang to let Spooky and the Ghostly Trios go, but Sigmondo insists on not doing so when he try to destroy those ghosts into the mystic abyss but the Witch Sisters used their powers to stop him and Jade told him that his battle is with her.
As Jade and Sigmondo begin to fight, they are punching each other and going through each other too. Jade begin to push Sigmondo down and made him press destroy lares bottoms and made everything going down, Casper, Kat and the gang begin to escape but Jade told them that she'll catch up and she will lose her supernatural powers, but Sigmondo said "The game is over, losers!" Casper told him that the game isn't over yet. Then Wendy use her magic wand to vanish Sigmondo and she did.
After Wendy defeated Sigmondo, she, Casper and the gang saw Jade's outfit on the ground and they all started to feel sad but they suddenly saw Jade who's not a ghostly human girl anymore, she's now a regular ghost like Casper, Spooky and the Ghostly Trios. But Casper and his friends said that Sigumondo wants to capture the ghost and wants to take over the world but Wendy uses her magic wand and defeated so he's gone.
After that, Jade decided to live with Casper's great-aunt, Spitzy and she told her that she teach Casper's uncles how friendly she's dose for them just like he did, Casper told Jade that he'll come visit her every time he's see her and Jade agree with him so she gave her brother a hug and thanks him and his friends for their advice so Casper and the gang returned to Whipstaff manor and they lived happily ever.