After an accident in the mysterious lab of maniacal scientist Dr. Simon Barsinister, an ordinary beagle unexpectedly finds himself with unimaginable powers and the ability to speak. Armed with a fetching superhero costume, Underdog vows to protect the beleaguered citizens of Capitol City and in particular one beautiful spaniel named Polly Purebread. When a sinister plot by Barsinister and his overgrown henchman Cad threatens to destroy Capitol City only Underdog can save the day.