The Scooby Gang, also known as Mystery Inc., are an amateur crime-solving group of friends. The gang consists of leader Fred Rogers, Daphne Blake, Velma Dinkley, Norville "Shaggy" Rogers, and his Great Dane Scooby-Doo. Together they travel the country looking for mysteries and debunking seemingly-supernatural occurrences as nothing more then criminal acts perpetrated by crooks.
Velma, along with the rest of the Scooby gang -- Fred Jones, Daphne, Shaggy, and Scooby -- meet Dean and Sam after the brothers have been drawn into their world. The Winchesters tag along to the reading of the will of Colonel Sanders, who has left money to Scooby.
At the reading, the lawyer announces to the gang and other beneficiaries that whoever stays in the haunted mansion all night would inherit the money. They all settle in for the night, the girls in one room and the men in another. Dean is about to eat his sandwich when they all hear a scream. Two of the other beneficiaries are killed and a ghost is seen. Castiel arrives and confronts the ghost. The Scooby Doo theme song plays at this point as the characters all run away from the ghost. They pop out of urns, and Dean tries to steal a kiss from Daphne.
After Shaggy is injured by the ghost, Dean tells the gang that ghosts are real. They have a hard time accepting this news and are very upset. Scooby and Shaggy set up an elaborate trap involving coconuts, iron chains, and a washing machine. The ghost comes in and it goes all wrong. The three of them get caught up in the trap. Plan B is to get it into the library. The ghost falls into the basement in a circle of salt. He can't get out. Then it turns into a little boy. The boy explains that when he died, his soul was tied to a pocketknife. Jay, from the pawn shop, found a way to control his spirit and use him to hurt people.
Dean promises that if he can get them home, they will release him from the knife. Dean tells the Scooby gang that Velma was right -- it was not a ghost. They come in to see the attorney Cosgood Creeps wrapped up in the circle of salt. Dean comes in with a suitcase with the cash; it is Confederate money -- worthless. Cas touches Shaggy's arm and heals it. Dean says goodbye to Daphne. Cas hugs Scooby and Shaggy. Velma shakes Sam's hand then pulls him down into her arms and kisses him.