"Hyde and Seek" is the sixteenth and final episode of The Mumbly Cartoon Show. It originally aired on ABC on December 18, 1976.
Lt. Mumbly is called away from the water cooler by Chief Shnooker for another assignment: a complaint about strange happenings at Dr. Seek's laboratory. Mumbly drives his beat-up, run-down jalopy over to the lab to find out what he's up to. Dr. Seek greets Mumbly at the front door and invites him inside; he reveals that he is creating a new soda-pop and is in the middle of a new experiment, and goes back to his lab. There he drinks his latest sample and is transformed into the insidious Mr. Hyde! Mumbly, who is waiting in the next room, is immediately snatched up and deposited outside by Mr. Hyde. Suddenly, the creature turns back to normal Dr. Seek, who is completely unaware of what has happened.
Mumbly once again knocks on the front door and greets Dr. Seek. While following Mumbly, he once again morphs into the insidious Mr. Hyde, who snatches the dog detective up, sticks him inside a paper airplane, and tosses him out the front door. Hyde transforms back into Seek when Mumbly flies back inside, but changes back to Hyde in time to snatch the paper plane from Mumbly sending him flailing into a closet. The Hyde creature searches in the closet but to no avail, as Mumbly is on the other side of the closet door.
Hyde turns back into Dr. Seek, who greets Mumbly and offers him a nice cool drink. Mumbly agrees, and Seek goes back to his lab, only to once again become Mr. Hyde, who makes yet another futile attempt to capture Mumbly. Then Hyde become Seek once more; Mumbly, hiding in a chandelier above, drops down to describe Mr. Hyde, but as his back is turned, Seek is Hyde once more, and Mumbly immediately turns tail! Hyde gives chase, but Mumbly has hidden himself in a cookie jar; just then, Hyde turns back into Dr. Seek once again, only this time, Mumbly actually witnesses the transformation! Mumbly confronts Dr. Seek with this knowledge, and Seek confesses; he was trying to develop a brand-new flavor for 6 super sodas, but all of the experiments proved to be one flop after another. On that, he becomes Hyde and then himself once more; the effects are only temporary, fortunately. Mumbly tells Seek he has excluded one important ingredient: the fizzle.
As Dr. Seek attempts to redo the formula, Chief Shnooker arrives to look for clues. He finds one of Seeks's super sodas, and, tempted, imbibes it and becomes a Hyde creature himself! The episode ends with both Dr. Seek and Shnooker taking turns becoming Hyde creatures and turning back into themselves, much to Mumbly's amusement.