Scooby-Doo is a live-action/CGI film based on the iconic Hanna-Barbera cartoon franchise. Its cast included Linda Cardellini as Velma Dinkley, Sarah Michelle Gellar as Daphne Blake, Matthew Lillard as Shaggy Rogers, Freddie Prinze, Jr. as Fred Jones and Neil Fanning as the voice of Scooby-Doo. These actors reprised their roles in the sequel, Scooby-Doo 2: Monsters Unleashed.
At a factory, the group are investigating the Luna Ghost in a typical case. Daphne (Sarah Michelle Gellar) has been captured, bound with jump rope and gagged with her own scarf. The gang trap goes wrong, but despite this the ghost is captured. The ghost is revealed to be a creepy janitor. After solving the case of the Luna Ghost, the members of Mystery Inc. argue over Fred's taking credit for Velma's plan and Daphne being told she always gets captured, and the group disband. After two years, Fred (Freddie Prinze, Jr.), who was the group's leader, becomes a celebrity; Daphne, who was the team's damsel in distress, is now a martial arts expert, and Velma (Linda Cardellini), the team's genius, is now a NASA employee, while Shaggy (Matthew Lillard) and Scooby (Neil Fanning), the group's dimwitted, but heroic backup, have now become beach bums.
Two years after breaking up, the gang are each invited to the remote amusement park Spooky Island. After an airplane flight to the island in which Shaggy falls in love with a young woman named Mary Jane (Isla Fisher), they meet the park's owner, Emile Mondavarious (Rowan Atkinson), who explains that something strange is happening to the visiting students.
Despite Fred, Velma and Daphne still refusing to work together—although Scooby and Shaggy are fine with their friends,—the five investigate. A man named N'Goo Tuana (Steven Grives) explains that the island's demons seek vengeance against Mondavarious for building the theme park, and a demon lures Scooby into the forest. Despite a warning from Voodoo Maestro, Daphne goes to a castle containing a closed-down ride, only to find the rest of the gang, who realize it is the obvious place for mysterious wrongdoing. Daphne finds a tetrahedron-shaped artifact called the Daemon Ritus, and Velma and Fred discover a classroom with a training video for non-humans. They all flee when confronted by N'Goo's henchmen, including a masked wrestler named Zarkos.
At the island's hotel, Velma reminisces about Scrappy Doo (Scott Innes), a little dog who was kicked out of Mystery Inc. due to his annoying behaviour. Just then, monstrous demons storm in and captures Fred, Velma, Mondavarious, and most of the guests after the attack daphne learned fred and velma have always been there for whenever she got kidnapped and the never gave up on her now she's not giving up on them because she realizes her friendship with them is special. Mary Jane calls the Coast Guard for help, but they do nothing, having already been possessed by demons. The next day, Fred, Velma, Mary Jane and more guests are possessed, and Zarkos captures Daphne and the Daemon Ritus. When Scooby falls down a hole due to him and Shaggy falling out because Scooby saw Mary Jane's demon who had possessed her in her eyes, Shaggy follows him into an underground hideout and finds the Daemon Ritus near a vat full of protoplasmic heads containing people's souls. When he pulls Velma, Daphne and Fred out of the vat, they return to their bodies. Daphne and Fred initially get mixed up with each other, but with help from the Daemon Ritus, the sleuths return to their original bodies and find Maestro, who explains that the demons can rule Earth for the next 10,000 years if their leader absorbs a "purely good soul". They realize that the pure soul is Scooby, and that the plans mastermind is Mondavarious, who arranged for Scooby to come to the island to trick him into being a sacrifice (Although Fred notes that this still fails to explain why he and the others were brought here).
Meanwhile, Velma has discovered that the demons explode in sunlight, which is why they need to possess the guests. Finally working together again, Mystery Inc. sets a trap to kill the demons by reflecting sunlight from a skull-shaped disco ball, but it doesn't work. Fred and Velma are captured, and Scooby's protoplasm is extracted from his body leaving it limp and presumably dead. In desperation, Shaggy attacks Mondavarious as he absorbs the protoplasmic heads, freeing Scooby and revealing Mondavarious to be a robot controlled by Scrappy Doo, hoping to gain his revenge on the gang after summoning them to the island to witness his triumph. Using his newly absorbed protoplasm, Scrappy expands into a giant version of himself, and another madcap battle ensues in which Daphne uses her martial-arts skills to defeat Zarkos, who falls into the hole and lands on the vat.
When the vat tips over, the protoplasmic heads return to their bodies and Daphne deploys the disco skull, which reflects sunlight streaming down the hole, killing the demons. Shaggy sucks the remaining protoplasm back out of Scrappy, who returns to his original appearance, Fred and Daphne share a kiss, and Shaggy finds the real Mondavarious trapped underground. After a public presentation in which Fred finally allows Velma credit for her work, the members of Mystery Inc. do their signature hands-together cheer. Scrappy, N'Goo, and Zarkos are arrested and taken away by coast guard helicopters, and the film ends as Scooby and Shaggy enjoy the 'all you can eat' deal that was used to lure them to the island.