The movie opens upon Shoeshine, a watchdog beagle who works as a bomb-sniffing agent of the police force; in the beginning scene, he is fired from the mayor's office for a bumble on the job. The very same night, he is kidnapped and injected with a strange elixir by a mad scientist, thus bestowing upon him superpowers. Shoeshine escapes, only to be adopted by a man who works as a security guard, an ex-cop, a widower and a father to a gloomy teenage boy, Jack.
Jack then discovers that Shoeshine has superpowers, and it becomes a secret between the two friends. Shoeshine, with the alias of Underdog, proceeds to do good around the city and saves people in distress; meanwhile, the mad scientist plots to capture Shoeshine and use his powers to take over the city.