In the bustling metropolis of Capital City, a beagle named Shoeshine is subjected to a genetic experiment led by the nefarious scientist Dr. Simon Barsinister and his bumbling assistant Cad. The experiment unexpectedly bestows upon the humble beagle a variety of superpowers, including incredible speed, strength, and the ability to speak. Following a fortuitous escape from the laboratory, Shoeshine is discovered by a security guard named Dan Unger and his son, Jack. Unaware of Shoeshine's newfound abilities, they adopt him and give him a loving home. Shoeshine struggles to adapt to his new life as a family pet while keeping his incredible powers a secret.

However, as danger looms over Capital City, Shoeshine realizes he cannot stand idly by. Driven by a desire to protect his new family and the innocent citizens, he adopts the superhero alias "Underdog" and vows to thwart any threats to the city. Meanwhile, Dr. Barsinister, obsessed with capturing Underdog to exploit his powers for evil, devises a diabolical plan that could put everyone at risk. With the help of a charming cocker spaniel named Polly Purebred, who becomes his confidante and love interest, Underdog must navigate the challenges of his dual life, fighting crime, and concealing his true identity from the Ungers and the rest of the world, setting the stage for an epic showdown between good and evil.