A laboratory accident gives an ordinary beagle named Shoeshine Boy (voiced by Jason Lee) unparalleled superpowers, including the ability to speak with a human voice. He is soon befriended and adopted by a lonely 12-year-old boy named Jack (Alex Neuberger). The two develop an even greater bond when the boy learns of the dog’s incredible powers and secret identity as the crime-fighting pooch named Underdog. Dressed in his very own superhero outfit, Underdog flies over Capitol City, protecting its citizens from unforeseen tragedies and keeping a close eye in particular on a beautiful spaniel named “Sweet” Polly Purebred (voiced by Amy Adams). When the dastardly, mad scientist named Simon Barsinister (Peter Dinklage) and his overgrown henchman Cad (Patrick Warburton) threaten to destroy the city, Underdog, truly “man’s best friend,” may be the citizens’ only hope.