Scooby-Doo/Best scene/Matthew Lillard/Shaggy Rogers/Neil Fanning/Scooby-Doo

The members of Mystery, Inc. solve the case of the Luna Ghost. However, long-time friction among Fred Jones, a glory hog; Daphne Blake, who is fed up with being kidnapped at every mystery; and Velma Dinkley, who is never credited for her ideas, finally boils over and the gang breaks up, leaving Shaggy Rogers and his dog Scooby-Doo heartbroken. Two years later, Shaggy and Scooby are invited to solve a mystery on the popular resort Spooky Island, owned by Emile Mondavarious. At the airport, the pair is excitingly reunited with the rest of Mystery, Inc.—Fred has become a popular author, Velma works for NASA, and Daphne has undertaken martial arts to avoid kidnappers. However, while Shaggy and Scooby are thrilled with the reunion, the other three still refuse to work with each other. On the flight over, Shaggy falls in love with a girl named Mary Jane, who loves Scooby Snacks like him, but is allergic to dogs. Upon arriving at the island, the gang meets Mondavarious, who claims the visiting tourists have been cursed into a brainwashed state. Velma attends a ritualistic performance hosted by N'Goo Tuana and his henchman, Zarkos, a famous luchador. N'Goo claims the island was once ruled by ancient demons that have been plotting their revenge ever since they were displaced when Mondavarious built the resort. Because of a misunderstanding when talking to a local voodoo priest, Daphne, Shaggy, and Scooby venture into the resort's ghost ride and meet Fred and Velma inside, where they split up to look for clues. Fred and Velma come across a strange school designed to educate inhuman creatures about human culture, while Daphne discovers a pyramid-shaped artifact called the Daemon Ritus. The gang flees to the hotel, but an army of real demons attack, kidnapping Fred, Velma, Mondavarious, and other tourists, while Scooby, Shaggy, Daphne and Mary Jane escape. The next day, Daphne is captured by Zarkos, and steals the Daemon Ritus. Daphne, Shaggy and Scooby learn that demons have possessed Fred, Velma and the other tourists. They encounter Mary Jane, but Scooby realizes she is possessed as well. Shaggy begins to argue, but is interrupted when Scooby falls through a hole into an underground chamber. While searching for him, Shaggy discovers a vat of protoplasm containing the souls of those possessed. He frees Velma, Fred, and Daphne's souls. Velma discovers the demons can be destroyed by sunlight, but Fred and Daphne become trapped in each other's incorrect bodies until the Daemon Ritus corrects this. They come across the voodoo priest, who explains that the demons, whose leader turns out to be Modavarious, will rule the world for the next ten-thousand years if a pure soul is offered as a sacrifice during their ritual. Shaggy realizes the pure soul is Scooby, who unwittingly accepts to be the sacrifice, having no idea what a sacrifice is. While curious on why Mondavarious invited them too, Fred, Daphne, and Velma decide to put the previous mystery behind them in order to help Shaggy save Scooby. The gang plot a trap to defeat the demon cult, but it fails and Mondavarious steals Scooby's soul using the Daemon Ritus. Shaggy then tackles Mondavarious, causing Scooby's soul to be released. As Fred and Velma confront the defeated Mondavarious, they find that he is wearing a mask. When Fred peels the mask off, they and Shaggy discover Mondavarious is a robot controlled by none other than Scooby's estranged nephew Scrappy-Doo, who was abandoned by the gang years ago after he urinated on Daphne and attempted to take over Mystery Inc. Using the absorbed souls of the tourists, Scrappy transforms into a monster and tries to kill the gang, explaining that he brought them to the island so he could get revenge on them. On the mountain top, Daphne battles Zarkos, finally kicking him into disturbing the ritual chamber freeing the souls they possessed, to which the demons are exposed to a disco skull's sunlight reflection and die. Shaggy rips the Daemon Ritus from Scrappy's body to free the rest and finds out that the real Mondavarious is found imprisoned in a small underground cell. Scrappy, N'Goo, Zarkos and all minions are arrested, while the reunited gang promise to forever solve mysteries. As the end credits begin to roll, Scooby and Shaggy are enjoying the "all you can eat" deal they were promised at the hotel. They challenge each other with eating chili peppers but scream as smoke emerges from the hotel.