Scooby-Doo 2: Monsters Unleashed is an 2004 Comedy horror film
Mystery Inc. (Fred, Daphne, Velma, Shaggy and Scooby-Doo) is attending the grand opening of the Coolsonian Criminology Museum, the premiere exhibit displaying the monster costumes of criminals they dealt with in past cases. However, the celebrations are cut short when the reanimated Pterodactyl Ghost attacks, controlled by the Evil Masked Figure, who boldly declares that Mystery Inc. will be destroyed. He escapes with his quarry and two monster costumes; the Black Knight Ghost and the 10,000 Volt Ghost.
Heather Jasper-Howe (Alicia Silverstone), a journalist, starts a smear campaign against the gang to discredit them, mainly stemming from a personal feud with Daphne. Velma finds a pterodactyl scale and analyses it, confirming it to be real. The gang conclude one of their old enemies is the masked man wanting revenge. Fred initially suspects that their old enemy Jonathon Jacobo (Tim Blake Nelson), who was the original creator of the Pterodactyl Ghost and knew how to create real monsters and bring them to life, but Velma quickly reveals to the gang that Jacobo drowned during an attempted escape from prison three years prior, so the gang guess Jeremiah Wickles (Peter Boyle), their first foe and the Black Knight Ghost's portrayer, and Jacobo's roommate in prison, is responsible.
The gang travel to Wickles' mansion where they find mysterious green footprints like the scale leading to the library. Fred, Daphne and Velma find an instruction manual on how to create monsters that once belonged to Jacobo, leading them to conclude that he passed it on to Wickles before his death and that Wickles is continuing his work; while Shaggy and Scooby are chased by the reanimated Black Knight but not before finding a note reading "Faux Ghost tonite" which they take as a clue. Daphne fends the knight off until the gang can escape.
Shaggy and Scooby sneak into the nightclub, the "Faux Ghost", where many of the convicted criminals they put away hang out, including Wickles who claims he has reformed from his criminal ways; but quickly they are recognized and thrown out. Velma learns the key ingredient to making monsters is a substance called "randomonium". Patrick Wisely (Seth Green), the museum curator, who Velma has a crush on, comes to aid the gang but when they go to the museum, they find the rest of the costumes have been stolen.
Shaggy and Scooby encounter the distressed Patrick, and then follow Wickles to the old mining town where the rest of the gang realize randomonium was mined. Wickles is proven to be innocent when the gang learns he plans to turn the mining town into an amusement park for children. He also reveals that he despised Jacobo during his time in prison because he has stolen his tater tots and got the lead in My Fair Lady, and thus has no reason to be continuing his work.
After being pursued by a Skeleton Man, Scooby and Shaggy take an elevator when they find a potion room called Clue-topia filled with glasses that contain colored potion (which don't taste very well) to transform people into monsters. Scooby and Shaggy discover a refrigerator, open it, and then turn into two cranky citizens, Shenzi (Wallace Foley), a buff man, and Flaps (voiced by Andy Roberts), a potion color mixer as a dog, after drinking each potion that are colored.
Flaps pours the blue into the green that makes it boil. Shenzi, refusing to be transformed back, throws the boiling potion as it explodes and the two are shoved into the wall. When the gang shows up, Shenzi and Flaps suddenly drink the blue potion, which transforms them back to Shaggy and Scooby. The gang are concerned that they have left on their own without them.
Reuniting, the gang find the Monster Hive, finding a machine which brings the monster costumes to life. Shaggy and Scooby mess with the machine's control panel, causing several monsters to be reanimated and unleashed. The gang flee with the control panel, as the Evil Masked Figure attacks Coolsville and Howe asks the gang to turn themselves in to save the citizens. The gang flee to their old high school clubhouse where they realize they will be able to reverse the Monster Hive's effects by altering the control panel's wiring.
Outside, Shaggy and Scooby lament on their habit of being clumsy and desire to be heroes. Captain Cutler's Ghost rises out of the nearby bayou, forcing the gang to flee in the Mystery Machine back to the mining town. After the song "Wanted Dead or Alive," Fred and Daphne fight the Black Knight and electrical 10,000 Volt Ghost, defeating them with a pair of jumper cables, Velma dispatches the Skeleton Men and Shaggy and Scooby defeat Miner 49'er using Scooby's farting gas to reflect the villain's fire breath back at him.
Velma, having eluded her pursuers, finds a shrine to Jacobo, actually made by Patrick who was Jacobo's student at one time, but is not the bad guy proving that by saving Velma's life. However, he is captured and dragged away by the Pterodactyl Ghost. Fred and Daphne run by as Velma reveals that she gave the control panel to Scooby and Shaggy and they go to find them before returning it to the Monster Hive.
After this, Shaggy and Scooby run into the Cotton Candy Glob which threatens to destroy them. Amazed at this monster, instead of running away as they usually do, they instead begin to eat the Cotton Candy Glob. During the process of being eaten, the Cotton Candy Glob begins to cry out "No! I'll give you cavities!".
The gang confront the Evil Masked Figure who has all of them held captive by the Tar Monster, who melts them in his traps, aside from Scooby, who grabs a nearby fire extinguisher to freeze the monster and makes his way to the machine and inserts the control panel, turning the monsters back to costumes. The Evil Masked Figure tries to escape but trips on a broken cat walk and is left dangling from it by his cape.
The authorities, press and Coolsville citizens arrive, where the gang unmasked the Evil Masked Figure as Howe - only to then unmask her as none other than Jacobo, alive and well. Jacobo survived his apparent "death" and was photographed outside the museum before it opened (he had been presumed dead a year before the museum's construction had even begun), found by Velma at the shrine.
Jacobo and Howe's cameraman Ned (Zahf Paroo) are arrested, whilst Mystery, Inc. are welcomed back by Coolsville with open arms. Velma and Patrick agree to go out again, Fred and Daphne remain at a peaceful relationship and Shaggy and Scooby stay best pals, though not before Scooby attacks Shaggy with a boom mike after mistaking him for a monster. The main cast dance with Ruben Studdard at the Faux Ghost in the ending scenes.
In a post-credits scene, Scooby is shown in a creepy hall playing the "Scooby-Doo 2" game on a Game Boy Advance. He then turns the game screen towards the camera, and says "Game Boy Advance secret code", showing a cheat code for the game he was playing.