 Scooby and the gang visit Seattle Underground to trap a red, hissing demon who has been menacing a construction site. . . . Blue Falcon and his inept mechanical sidekick, Dynomutt the Dog Wonder, battle the evil Swamp Rat, who wants to turn all of Big City into a swamp and then steal everything in sight. Meanwhile, Scooby and the gang are after the swamp ghost; but it turns out the daring duo and the gang are after the same quarry.