After an accident in the mysterious lab of mad scientist Dr. Simon Barsinister, an ordinary beagle finds himself with extraordinary powers…and the ability to talk! Armed with a fetching superhero costume, Underdog vows to protect the beleaguered citizens of Capital City, particularly a beautiful spaniel named Polly Purebred. When a diabolical plot by Barsinister and his overgrown henchman Cad threatens to destroy Capital City, only Underwear…(oops!)…Underdog can save the day.