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