Plot

The gang is pulled out of retirement to investigate a mystery on a familiar zombie-infested island.

The Mystery Gang lure a werewolf into a trap and catch the wrongdoer: Young Man Withers. Soon after cleaning the Mystery Machine, the gang capture more monsters including the Snow Ghost, the Creeper, and the Spooky Space Kook, who are the entire Withers family. But as the gang congratulate themselves, the Mystery Machine goes backwards, prompting Fred to give chase before discovering that this was just a dream. As this turns out, Fred sold the Mystery Machine and they are retired from mystery solving, thanks to the local Sheriff. Fred decides to come out of retirement after seeing a news report, though Shaggy and Scooby enviously refuse, saying they have been constantly taken for granted into all their mysteries. The pair make Fred, Daphne and Velma promise never to solve another mystery. The gang decide to have a vacation and soon see a television program featuring Elvira, who announces that Shaggy Rogers is the winner of a tropical vacation, much to Fred, Daphne and Velma's confusion and Shaggy and Scooby's delight. They do have a brief encounter with the Sheriff who reminds them not to do any more mystery-solving.

On the way to the island via ferry, Fred, Daphne and Velma attempt to keep their promise as Shaggy and Scooby relax. After arriving, two greeters frightfully order the gang "Get Out" but get humorously ignored by the gang as they go inside a van to the resort, with the two greeters saying "Tourists" into an annoyed tone. On the way, a clawed figure slashes a palm tree causing this to block the road, forcing the gang to go to the resort by foot while the van driver stays to repair his van. Arriving on Moonscar Island resort, they are reminded of Moonscar Island where they solved a mystery involving the zombies and Car Creatures. Inside the resort, they meet the hotel manager Alan and the hotel staff, composed of Mona, Linda, Jack and Bugbite Stubbs, who resemble the people the gang previously encountered on Moonscar. While enjoying the accommodations, a few zombies surprise Shaggy and Scooby, who run away and hide inside a cabinet, which ultimately leads to a cavern, complete with cat statues and monuments.

Shaggy and Scooby soon return to the hotel and barricade the door, as a group of zombies approach the hotel. Shaggy and Scooby give their okay expressions to the others to solve mysteries again and the gang arrange a trap for the zombies. But the clawed figure, a black werecat, knocks over a candelabra, setting fire to a curtain, which then causes the sprinkler system to douse the fire and melt the zombies' faces to reveal that they were the hotel staff into disguise. The gang discover that they are actually on Moonscar Island, which is doubling as a filming location. Alan confesses to the gang that he's a film director known as Alan Smithee, who wanted to make a scary movie after looking into Velma's unsolved mysteries blog. He also revealed that he sponsored the vacation for Shaggy to win, but states that he has knows nothing of the black werecat. Despite everything that happened, the gang offered to help Alan finished the movie. As filming continues, the gang meet Seaver, a stunt double for Fred, who has a monster truck version of the Mystery Machine. As the sun goers down, Velma heads into the forest wearing night-vision goggles but runs away when she sees a cluster of cats' eyes.

After Velma orders the others to make a run for their lives, they see a harmless-looking being kitten coming out of the bushes, startling Scooby. Just as Shaggy assures Scooby that there's nothing to worry about, the kitten (fully grown into an instant) turns feral and roars like a tiger, scaring Shaggy as more cats attack. After the movie crew (except Alan and Seaver) hide, three werecats appear. As the gang and Seaver run for the ferry, Alan crazily burns this. Acting on impulse, Fred rescues the Mystery Machine from being burned with the ferry. Fred orders the other to get inside, but one of the cat creatures attempts to grab Smittee's medallion, which is Simone's pendant. The gang save Alan and the pendant and the cats and cat creatures pursue them back to the resort. After a wild chase, the gang hide inside the cabinet Shaggy and Scooby were hiding inside, leading to the cavern where the werecats attempted to drain the gang's life force. They notice that there are a great deal of holes inside the cave, which gives Velma an idea. The three cat creatures enter the cavern, find the medallion, and place this on the pedestal into the light of the harvest moon. The light is reflected to a specific spot on the ground were the cat creatures dig until they find a chest that appears to be filed with gold coins, though its contents turn out to be Scooby Snacks covered inside gold foil. The cats are attacked by the gang, disguised to resembled zombies, causing them to become scared and hide inside one of the cave's holes.

The gang unmask the creatures, revealing them to be the greeters and the ferry captain. The trio intended to find Moonscar's treasure, using cat whistles to control the island cats to deter anyone who were getting too close. The fourth werecat (who attacked the van and attempted to set a fire inside the mansion) is implied to be a real monster and remains at large. After the police take away the wrongdoers, Alan despairs that his movie doesn't have a finale. The now docile cats soon enter the cave and dig a spot into the ground with Scooby's help, revealing another treasure chest. Alan opens this and sees real gold inside, and declares that he's done with movie business and is retiring with all the gold. The gang declares that they will continue to solve mysteries. Though the sheriff arrives and berates the gang for solving mysteries again, he relents, allowing the Mystery Gang to return. Along with the six cats who dug up Moonscar's treasure, Shaggy and Scooby eat the gold foiled Scooby Snacks. Elvira informs the viewers about the movie Teenage Zombies and the Island of Doom, including a trailer and outtakes.