The Mystery Inc. is heading to the Big Halloween Movie Festival, until they run into Gary Coleman, who is on the side of the road asking for a lift. They let him in the Mystery Machine, and shortly after they notice that they are running out of gas near to an old abandoned amusement park. They decide to pull over to check it out, as is a tradition of the Scooby gang, but Gary Coleman, who is oblivious to the fact that exploring spooky abandoned places is their favorite hobby, gets a tank of gas and fills up the van's tank, much to the crew's annoyance.
They fake gratitute and continue their way, now finding a creepy steamship. Fred purposely flattens the back tire of the van by going over a rock to give them a excuse to stop and investigate it, but Gary Coleman inconveniently has a spare tire. He fixes the van, and they are forced to continue their path.
They now come across a spooky old castle, and Fred decides to ram the van into a tree, making it completely unable to continue and be fixed by Gary, giving them no choice but to stop and investigate the place. They begin checking the surroundings of the castle and Gary immediately notices a graveyard full of open graves, while Velma notices a sale sign, giving them another mystery. The castle, whoever, is guarded by a drawbridge surrounded by a moat inhabited by a shark, which is actually Jabberjaw from the namesake Hanna-Barbera cartoon.
The castle is owned by David Cross though, who greets the gang and wishes them a Happy Halloween by throwing cans at them, which all hit Gary Coleman in the head. He compliments them on their "costumes", and introduces himself as the famous David Cross from a 40-year-old groundbreaking and critically-acclaimed comedy show, but none of them ever heard of him. He lowers the drawbridge and allows the gang to get in.
He welcomes the Mystery Inc. at the reception, similar to a hotel, and offers to fix their van in the morning. Velma notices a wall full of pictures of celebrities that have stayed there in the past. Gary says that it used to be a resort for the rich and famous, but he now has to sell it because business dropped off since someone started playing pranks, with eating guest's brains being one of them. This scares Shaggy, Scooby and Gary right away, and David tries to reassure them by asserting that there are no zombies there, further scaring them instead as he had now brought up zombies.
He introduces his assistant Mr Shifty, which comes in saying that he has prepared the zombie barricade, putting David Cross in a rather awkward situation. He pretends to not know what he is talking about and inadvertly starts making painfully obvious that Mr Shifty is behind the whole zombie plot, but he and the whole gang but Gary Coleman fail to see this. As they walk through a hallway, a trapdoor opens beneath Gary Coleman and he falls, unknowingly to the gang.
Daphne and Velma arrive to their room where they are being watched by a painting. Just as they are about to undress, David Cross comes in and offers them some towels, which they accept. He casually opens the painting, revealing a hidden room where Shifty was watching them all along, further incriminating him. He asks him to bring them some towels and he awkwlardly leaves the room.
At the kitchen, Shaggy and Scooby are searching the place for food until David Cross arrives. He takes notice of Scooby being a talking dog and is amused by his speech pattern. Just as he's asking Scooby to say his name in the same way of his famous catchphrase, a zombie appears from behind. They try to warn David but he fails to notice and is grabbed by it, and Scooby and Shaggy flee the room.
They later meet the rest of the crew to tell that David has been kidnaped by a zombie, and they also finally notice that Gary Coleman is missing. David then turns up in the room with his clothes all ragged, but he still denies the zombie's existence. They decide to search for Gary and split up, but David takes Fred's role and divides the groups up, much to his chagrin, with the aggravating factor that he had put Daphne to search the dungeons alone with him. They argue until David pulls a book on a shelf that opens a trapdoor where he falls along with Daphne, with Fred following him along with Velma.
Shaggy and Scooby are then searching for Gary somewhere in the castle until they find Shifty in the zombie suit, who had failed to remember to put on the mask. He tries to scare them away, but even so they still fail to realize that he is the zombie. He notices that he forgot to wear the mask and leaves to put it on. He then comes back with the full costume and Scooby and Shaggy now run away in fear.
Some time later the group consisting of Daphne, Fred, Velma and David are searching for clues, until they find a set of footsteps that end at a wall. They conclude that there must be a secret passage, but David ruins the mystery for everyone by pointing where the hidden switch is. Fred asks David to stop ruining the surprises for everyone and he unwittingly reveals that there is another secret switch in the room. Fred asks him to at least not tell them where it is, and he eventually finds it on a bookshelf.
In the next room they find an old egyptian sarcophagus, from where a mummy comes out and starts heading towards them. They don't even acknowledge it as a threat however, and start making bandage-related puns. David fails to think of one and instead misses the point by making a "change the light bulb" joke, but the mummy grabs him before he can finish it. Velma moves in to rescue him and beats the mummy to a pulp.
David thanks Velma for saving his life, apparently having fallen in love with her, and Scooby and Shaggy suddendly appear. They finally figured that is Shifty in the zombie suit and claim to have solved the mystery, until Gary Coleman finally turns up, now with a greenish skin tone. He claims to be undead now, but the group fails to understand what he's saying. They finally annoy him by using his famous "whatcha talkin' 'bout" catchphrase, with him claiming they are reducing his life to it, and a chase ensues.
The chase continues until they bump into the band Big Bad Voodoo Daddy, who appearently are acquaintances of the Mystery Inc. They explain that they came to the castle to record a new album, but a mutant zombie made them undead and now they pretty much eat brains. David introduces Gary to the band, who is also a fellow zombie. One of the members asks if they are ready for the musical chase scene, and the zombie shows up again, and said chase scene begins.
The zombie is then caught by Mr Shifty, highly contradicting what was established before, and the musical chase scene ends. The zombie is unmasked and is revealed to be Jabberjaw. He reveals his motivations to be eliminating the Scooby gang, as he's envious of them because his show also consists of teenagers with a comedic sidekick, yet he lives in their shadow. Scooby then proceeds to say his famous "Scooby-dooby-doo" catchphrase. Daphne also points that he forgot to say the meddling kids line and he refuses to do so, but then unintentionally says it.
Daphne apologizes for the whole group to Mr Shifty, but he then reveals to be actually Mark Hamill. He says he has been working undercover on this case for a long time, and thanks the gang for the help. With the mystery solved, David tries to make the moves on Velma, but she rejects his advances. Gary then interrupts to say that this case doesn't make any sense at all, pointing various parts of the plot that are still not answered, such as why was Mark Hamill dressed as Mr Shifty dressed as a zombie, who was the mummy and why is he still undead when the zombie was Jabberjaw all along, but he's interrupted by Scooby, who jumps on him and starts licking him until the episode ends as everyone laughs.