After Casper and his best friend, Kat Harvey took on a boat ride, Casper's childhood friend, Jimmy Bradley from Deedstown came to have fun with him and Kat, then Casper's scare school friends, Ra the Mummy and Mantha the Zombie joined in with him, Kat and Jimmy. All together, they have fun until Snivel came and warn the kids there's a danger monster in Treasury Island and he needs their help to save them.
Later, the gang have to see the others because Casper's arch-enemy Kibosh has send Snivel to capture the kids and send them to the Treasury Islands, he also told him, if something happened, send Wendy the Good Little Witch and Hot Stuff the Little Devil to help Casper, Kat, Jimmy, Ra and Mantha. So they have to get Wendy who is on the broomstick witch her best friend, Peter Von Trapp and Hot Stuff immediately. Meanwhile in America, Peter and his mother FBI Agent Miranda Von Trapp are having so much fun with Wendy and the Witch Sisters (Gerti, Gabby and Fanny), but they have visitor, Snivel and he told Wendy and Peter that he needs their help, but 17 year-old girl named Mia Risley and her adopted father Nigel Risley a CIA agent told Miranda and the Witch Sisters because Casper and the gang needs help. And Miranda and Peter told them that Kibosh has send Snivel to get him and Wendy to help Casper and his friends, so the others go to warn Casper and his friends.
Meanwhile, Snivel came to Hot Stuff the Little Devil and his family, father, mother and brother, and he needs their help, but Hot Stuff didn't realizes that Casper and the rest will be help him and Wendy. Snivel told Hot Stuff to go meet with Wendy and Peter if they find Casper, Kat, Jimmy, Ra and Mantha at the treasury island because there's a mystery monster causing everything in the island, they will found everything. Hot Stuff told his family to find Dr. James Harvey, Miranda, the Witch Sisters and the Ghostly (Stretch, Stinkie and Fatso), while he find Wendy and Peter, and then they will find Casper and his friends.
At Treasury Islands, Casper, Kat, Jimmy, Ra and Mantha have found their way there, but they met a giant and it was Hot Stuff's best friend, Stumbo and he was lost in Treasury Islands. Casper and his friends ask him that there's a mystery in this island and they need his help, Stumbo would love to help them so he gives them a ride on his shoulders and then they have to do something because Stumbo was Hot Stuff's best friend. Later, Casper and his friends along with Stumbo who have to get a clue for his friends and they found Baby Huey, Little Audrey, Herman and Katnip.
Baby Huey, Little Audrey, Herman and Katnip told Casper and his friends that they have the same problem as they do, so they didn't realizes that Snivel is resurrected from death and brings a giant baby duck, a little girl, a mouse and a cat to the Treasury Islands where they can be safe. Casper told Baby Huey, Little Audrey, Herman and Katnip to come along with him, Kat, Jimmy, Ra, Mantha and Stumbo because there's a mystery in Treasury Islands. Baby Huey, Little Audrey, Herman and Katnip joined Casper and his friends on a mystery in Treasury Islands. Meanwhile, Nigel and Mia were helping Miranda, the Witch Sisters, Dr. James Harvey and the Ghostly Trios looking for his daughter and their nephew, Kat and Casper. But Peter ask his mother what about the plan to find the Treasury Islands.
The Ghostly Trios have known about that Islands long time ago before they and Casper met the Harvey's family, when their cousin, Hairy Scary took Casper to the island, they were thought if they can play music together, but Hairy Scary can't stand playing music so he hit Casper on a head with his guitar and then he chases him before Hairy can do Casper next, the Ghostly Trios came and they are not happy with Hairy Scary for mistreating Casper, the Trios told him that they can never speak to him again and then they fly Casper away from that hairy ghost. But Casper's uncles would be very scared if their nephew would meet Hairy Scary again, and he will get in trouble again, so they, Dr. Harvey, Wendy, Peter, Miranda, the Witch Sisters, Mia and Nigel have to go to Treasury Islands immediately.
Back at Treasury Islands, Casper and his friends have meet the Voodoo Monster the evil spirit of Treasury Islands, Casper think Voodoo Monster's face look familiar. Then suddenly, Hot Stuff came and realized that his giant friend, Stumbo is with Casper, Kat and the gang, Hot Stuff and Stumbo hug each because they haven't seen each other for a long time time. Fortunately, they were became best friends again, but Voodoo Monster can't stand that so he called his witch doctor friend to shrink Stumbo very tiny and he did. Hot Stuff demand that Voodoo Monster to tell his witch doctor friend to change Stumbo back to his giant self again, but Voodoo Monster won't, he just want Casper.
Herman and Katnip are mad because Voodoo Monster wanted Casper, so Katnip used his clew to scratch Voodoo Monster but it was going through, Little Audrey try to punch Voodoo Monster but it was going through too, then Baby Huey begin to punch Voodoo Monster's face but the mask came off and he wasn't a monster, it was a hairy ghost with red nose. The Voodoo Monster turn out to be the Ghostly Trios cousin, Hairy Scary who has scared any people in the Islands, after the Ghostly Trios left with Casper for what had Hairy done to him, he, Casper have time traveling the prehistoric period with Maxi, Mini, Nerdly and Fango, he and Casper helping the orphans to save Halloween and Christmas time with Yogi and the gang, but here in Treasury Islands it was no fun when Hairy did something bad to Casper before the Ghostly Trios told that hairy ghost that they are not going to speak to him again.
Then suddenly, Spooky and his girlfriend, Poil came to rescue Casper, Kat, Jimmy, Ra, Mantha, Hot Stuff, Stumbo, Baby Huey, Little Audrey, Herman and Katnip from Hairy Scary, then Buzzy the black crow show up too and he, Spooky and Poil help Casper and friends escape so Hairy sent those skeletons to chase them, but Spooky told Hairy Scary that he have to stay away from Casper before the Ghostly Trios will be angry at him again, but Hairy just wanted Casper so he captured Spooky, Poil and Buzzy and then he decided to do something bad to them.
After that Casper and Friends are safe but they lost Spooky, Poil and Buzzy, so they need to find help immediately. During this, Wendy, Peter, the Ghostly Trios, Dr. Harvey, Miranda, the Witch Sisters, Mia and Nigel took a sailboat to Treasury Islands, they find Casper and the gang whose almost defeat Voodoo Monster who's turn out it was Hairy Scary who had captured Spooky, Poil and Buzzy, the Trios are angry because Hairy Scary was behind the mystery of Treasury Islands, so they told their cousin to put them out of there. But Hairy review on not doing so he begin to capture Casper, and then use the giant fan and blows them away.
Later Wendy and others saw Casper's friends uncles and told them that Hairy has got Casper and takes him to Winnifred Witch and Screech Ghost and they are all doing something bad to Casper. Stretch got an idea so he told Kat, Wendy, Dr. Harvey, Miranda, Peter, Gerti, Fanny, Gabby, Fatso, Stinkie and others if they have to save Casper, Spooky, Poil and Buzzy and get Stumbo back to his giantself again, they must warn everyone and tell them about Hairy's giantself will be a same size like Stumbo was, then Kibosh and Snivel came with Jimmy's dad, Graham Bradley along with the angels officers, Mini and Maxi whose can help Kat, the Ghostly Trios, Dr. Harvey and others to save Casper, Spooky, Poil and Buzzy and changed Stumbo back to his giantself again.
Meanwhile at Hairy Scary's new place, he has takes Casper, Spooky, Poil and Buzzy because he has steals Stumbo's giant form. He told Winnifred and Sceech that he has bringing them ans Winnifred and Screech told that there's is a nice work, but suddenly Casper's allies and familly came to rescue them. Kat told Dr. Harvey, Miranda, Stumbo, Mia, Nigel, Stumbo, Graham, Mini, Maxi, the Witch Sisters and the Ghostly Trios to destruct Hairy, Winnifred and Screech while she, Wendy, Peter, Hot Stuff, Jimmy, Mantha, Ra, Baby Huey, Little Audrey, Herman and Katnip free Casper, Spooky, Poil and Buzzy, so they have find them. But Stumpo gets his giant size back from Hairy, then he got mad so he call his witch doctor friend again but the Witch Sisters stop him to save Stumbo.
Casper told Hairy Scary if he ever try to harm his friends and family, he have to catch him first. So Hairy grabbed Casper once again and screams at him, but Mantha use her head and throws it to hit Hairy and he drops Casper to the ground and landed unconscious. Then Maxi and Mini told Hairy Scary that he's under arrested for behind the mysterious of the Treasury Islands and than Kibosh and Snivel begin to take Hairy's haunting license including Winnifred and Screech's haunting license and then Graham use his creature catcher vacuum kenmore to vacuuming Hairy, Winnifred and Screech.
After that, Wendy, Peter and Kat wakes Casper as he was awake and he's okay. Snivel apologize to Casper, Kat and the gang for sending them to the Treasury Islands and they for give him, so Kibosh and Snivel disappear with their powers. Later the gang thanks Casper's angels for arresting Hairy for stealing Stumbo's giant form and Graham for capture Hairy, Winnifred and Screech so he told his son, Jimmy that it's time to come home in Deedstown, but Maxi and Mini returns to the future. The Ghostly Trios are so glad that their nephew is okay so everyone returned home and they lived happily ever.