An inventive criminal makes a formula that can turn anyone into an evil monster.
The gang got involved in the case because they thought they were dealing with their old enemy, the Ghost of Mr. Hyde (or perhaps a new criminal using that identity). They set a trap for him, but ended up catching Dynomutt instead. Dynomutt's owner, the superhero Blue Falcon, briefed them on the real situation; this was not a man pretending to be a ghost, but a villain with a split personality who could transform others into monsters. He accepted their offer of help with the investigation.
They went together to City Hall to discuss the case with the Mayor. They wondered at Mr. Hyde's motives, since he only transformed museum security guards into monsters and had them try to make off with the Crowl Jewels; he took nothing for himself. Fred, Daphne, and Velma went back to the museum to search for clues. Blue Falcon, Dynomutt, Shaggy, and Scooby-Doo went to the Orville Research institute to consult Professor Orville about an antidote. They didn't realize Mr. Hyde was outside the window and had overheard their plans.
At the Institute, Professor Orville was confident he could create an antidote, until Hyde came to the door pretending to be a telegram delivery man. When Orville opened the telegram, Hyde gas escaped and he was transformed into another monster. Shaggy, Scooby, and Dynomutt hid in a file cabinet when this happened. Hyde presented his demands: that he be made mayor by the end of the week or everyone in Big City would become Hydes. While Shaggy and Scooby watched, Dog Wonder tried to restrain the new Hyde and turned into a motorcycle instead, running into and capturing both Blue Falcon and the new Hyde. The original Hyde escaped.
Hunting around the Institute's natural history display for Hyde, Dog Wonder and Scooby-Doo pounced on a paleolithic cave man figure instead, and got covered in rough brown hair. Dynomutt thought the fur-covered Scooby-Doo was Mr. Hyde and sucked him up with his vacuum cleaner attachment, along with everything else in the room. Everything had to be put back together before they went after Hyde again.
Mystery Inc. and Blue Falcon regrouped at the Mayor's office. Fred showed a clue they found at the museum, a rubber glove dropped by Hyde. Dog Wonder ran an analysis on the glove and found that it belonged to Willie the Weasel, a third-rate second-story man who dropped out of sight two years before. They had cracked the identity of Mr. Hyde.
The Mayor refused to give in to demands of Hyde to be made mayor himself, so Hyde tried to carry out his threat to make everyone Hydes by driving around and spraying his formula from his car. The many Hydes created this way went on a crime spree. Blue Falcon couldn't keep up with all the Hydes. He told the Mayor they might have to give in to Mr. Hyde's demands, as part of a plan to capture him.
When Mr. Hyde took over the Mayor's office with Blue Falcon and Dynomutt watching, he tried one more trick to consolidate his power: he sprayed Blue Falcon and Dynomutt with his formula. They turned into Hydes and joined in his crime wave.
Mr. Hyde took them back to his old headquarters to get more Hyde formula to extend his empire to other cities. They asked him how he would get to other places to do this, since his Hyde face would be recognized. He demonstrated his antidote which turned him back to Willie the Weasel. That was all the Blue Falcon was waiting for - he and Dog Wonder had avoided breathing in the formula with concealed filters and applied their own instant Hyde disguises.
The Weasel still had a trick up his sleeve. He escaped out an emergency trap door and activated automatic steel plates to lock the superheros in. Dynomutt managed to break them out with his battering ram attachment.
Blue Falcon called in Mystery Inc. to the Mayor's mansion, where Hyde was collecting his loot before getting out of town. The gang went out back to set up a trap. They looked in the window to see how thing were going and saw Dog Wonder use a girl-dog puppet to capture Hyde's dog Snitch and get the antidote from him.
Hyde went out the front door, avoiding the trap in back, and tried to drive away. Blue Falcon stopped his car with a grappling hook attached to Dynomutt, who had drilled himself partially into the ground to provide a solid anchor.
The gang and Blue Falcon reported their success to the Mayor. Shaggy said there was one loose end he had forgotten, which turned out to be the trap in the back of the Mayor's mansion. Dynomutt fell into it again, and he activated all sorts of random gadgets trying to get himself out.
- Mystery Inc.
- Dynomutt (first appearance)
- Blue Falcon (first appearance)
- Mr. Hyde (single appearance)(transformation)/
- Willie the Weasel (single appearance)/
- Snitch (single appearance)(no lines)
- Hyde Monsters (single appearance)(transformation)/
- Miscellanous Big City citizens (redeemed)
- Big City
- Big City
- The Scooby-Doo!/Dynomutt Hour: The Complete Series DVD set released by Warner Home Video on March 7, 2006.