While walking home with a pizza, Scooby and Shaggy see a violin case fall out of a speeding sedan; opening it, they find it packed with new twenty dollar bills. Shaggy goes to phone the others while Scooby stays behind and guards the violin case. The sedan returns and a limping girl dog appears. Scooby drops the violin case to go help the dog; while he's occupied a mysterious figure grabs the violin case. Then the car and the dog disappear.
Freddy and the others show up, and they all look about for clues and find a puppet control. Examining the circumstances, the gang deduces that there must be a gang of counterfeiters on the loose. They go to the local theater, which is showcasing Pietro's puppets, under the pretense of getting an audition for their performing dog. The doorman, Pop, is playing with a puppet but claims only to be an amateur. While he puts the puppet away, Shaggy and Scooby find a new twenty dollar bill; after leaving, Fred nad Velma decide that the bill is genuine, a plant to lead them off the track. They return to the theater and sneak past the sleeping doorman. As they search the stage area, a cloaked figure watches them from the stage and tries dropping stage sets and sandbags on them. Both Shaggy and Velma, in different parts of the theater, catch sight of the figure, and they chase it down a prompters box and under the stage, where they find empty violin cases. Lured back to the stage by the sound of an organ playing, they are menaced by Viking and pirate figures. Scooby spots of the girl dog puppet and catches her. Inside its stuffing is the front engraving plate for a twenty dollar bill. But when they go to wake the doorman, they find that he's a lifesize puppet. Before they can leave, a bird swoops in and grabs the plate.
The gang returns to the room with the violin cases; behind a harp case they find a secret room with a printing press. The cloaked figure appears and chases them through a tunnel, up a ladder and back on stage, where they deduce that the figures menacing them are lifesize puppets. Peering up into the rafters, they spot the puppet master. Scooby chases and corners the figure. He is unmasked as Pietro the puppeteer (a.k.a. Pop the doorman), the man behind a mammoth counterfeiting operation.