There is a commotion at the Capital City Town Hall due to a false alarm initiated by an intrepid police bomb squad beagle. This beagle, however, is met with ridicule and is turned away, only to be kidnapped by the nefarious Cad Lackey and taken to the sinister laboratory of Simon Barsinister. Barsinister, disgruntled with the rejection of his proposal for genetic testing, seizes the opportunity to experiment on the beagle with his latest serum, believing it will prove his theories valid. However, the beagle manages to break free, unleashing chaos in the lab that ultimately leads to a fire and an exposure to a cocktail of chemicals—this mix alters his DNA and bestows him with extraordinary superpowers.
As he escapes the lab, the beagle cleverly avoids a pack of goons led by a menacing Rottweiler named Riff Raff. Despite being struck by a car driven by Dan Unger, the beagle, now nicknamed Shoeshine, survives the incident. Dan, looking to help the beagle, brings him home. After a humorous and somewhat endearing moment where Shoeshine licks Dan's shoes, the veteran police officer decides to give him a name—Shoeshine. However, this new friendship introduces a bit of rivalry as Dan's teenage son, Jack, begins to feel overshadowed by the affection and attention that Shoeshine receives. Following the tragic loss of Dan's wife, he had stepped back from his police duties to devote more time to Jack, complicating their already delicate relationship.
In an amusing twist of events, when left alone at home, Shoeshine inadvertently creates quite a mess due to his unawareness of his newfound abilities. To both their surprises, upon Jack's return, Shoeshine apologizes—an event that serves as a catalyst for their burgeoning friendship. They form an unexpected bond, united now by their mutual crush on Jack's friend Molly and her dog Polly. As Jack and Shoeshine explore Shoeshine's superhuman capabilities, they discover his extraordinary strength, incredible speed, and enhanced senses of hearing and smell.
One day, after realizing he can fly, Shoeshine leaps into action to rescue Molly and Polly from an armed robbery. Upon returning home with Jack, Shoeshine learns the importance of keeping his special abilities under wraps. However, cunning Cad appears, intent on capturing Shoeshine. After various escapades, including disguising himself as a fish to foil a robbery and donning a striking blue cape with a shrunken red sweater, Shoeshine adopts the heroic alias Underdog, becoming the city's beloved superhero.
The stakes rise as Barsinister, frustrated by his inability to replicate the serum's effects, enlists Cad to obtain a DNA sample from Underdog. Although Cad fails to capture him, he manages to snatch Shoeshine's collar, gaining access to his true identity. In a twisted turn of events, Barsinister and Cad kidnap Dan, using him to lure Shoeshine into a trap to retrieve his DNA for their sinister plans. As tension escalates, Barsinister also concocts a super pill for three trained German Shepherds, hoping to match Underdog’s powers.
In a gripping sequence of events, with the family stranded in the sewers because of Barsinister's machinations, Dan, relying on his police training, orchestrates their escape. Armed with a hypodermic needle containing an antidote, they set off to confront Barsinister. Meanwhile, Molly and Polly bravely pursue Cad after he captures the mayor during a chaotic scene at City Hall, planning to attach a mind-control bomb to the rooftop.
Upon smelling the explosive, Shoeshine overcomes his self-doubt and courageously infiltrates the building. However, he's soon confronted by the villainous Cad, who captures Polly and Molly. In a twist, during a fierce tussle with Barsinister, Shoeshine accidentally inhales a super pill, regaining his powers in the nick of time. A thrilling showdown unfolds as he rallies the German Shepherds to turn against Barsinister, showing the true meaning of friendship and loyalty.
With a final determination, Underdog emerges heroically on the rooftop, saving Molly and Polly while simultaneously defusing the bomb. Dan, reinstated as an officer, arrives just in time to inject Barsinister with the antidote, leading to the villain’s capture. In a nail-biting climax, just before the bomb detonates, Shoeshine manages to bury it deep underground, only to be caught in the blast—leaving everyone to believe he has perished. But amid tears and despair, Underdog triumphantly returns, eliciting joy from the crowd. As the legendary hero, Shoeshine resumes his vigilant watch over Capitol City, proving once again that heroes come in all shapes and sizes.
