The film opens with a slight introduction from the main hero of the film, Underdog who introduces the audience to himself, his arch-nemesis Simon Barsinister, and Barsinister's henchman Cad. The film then enters into Capital City Town Hall, where a beagle on the police bomb squad sets off a false alarm after smelling a large ham (in some boxes near where the mayor was giving a speech). After being shunned by his fellow police canines, the beagle leaves in shame and is abducted off the street by Cad, who takes him to Simon's lab. Simon is bitter against the mayor, who has turned down his proposal for a type of genetic experimenting for the city, and plans to prove him wrong by testing his new serum on the beagle. However, Underdog escapes and runs amok in the lab, causing a fire and accidentally becoming exposed to a variety of DNA concoctions and chemicals that Simon developed, which modifies his DNA and gives him the strength to break through the metal door to the lab. After witnessing his work's "success" Simon is stunned when a burning monitor falls onto him, and Cad rushes to his aid.
Later, the beagle avoids an encounter with a large Rottweiler named Riff Raff and his lackeys. He is later hit by a car belonging to Dan Unger. However, thanks to the serum he absorbed, the beagle is unharmed and taken home by Dan, who names him Shoeshine the next day, after having his shoes licked by him. Dan introduces Shoeshine to his son Jack, though the boy is less than pleased by Shoeshine's presence, thanks to a wedge between father and son after Dan retired from the police force to spend more time with Jack, only to never actually get the time to be home with him. Jack leaves for school and Dan for work, leaving Shoeshine at the house alone, where he, still unaware of his new powers, accidentally makes a very large mess of the place. Jack comes home to the destruction and begins to clean up, complaining, to which Shoeshine claims it was an accident, leading to both of them realizing (to their shock) that Shoeshine can speak human language. Jack initially tries to get away from Shoeshine, but after they reach a park, the two begin to talk and bond a little when a friend of Jack's, Molly, walks over and introduces them to her Cavalier Spaniel, Polly, whom Shoeshine is smitten with on sight. After the girls leave, Jack and Shoeshine experiment with his powers through various habits of dogs.
Far away, Molly and Polly are being mugged by two men, and Shoeshine hears Molly's call for help and rushes to their aide, running faster than he has ever before, and discovering he can fly in the process. He rescues the two without them seeing him, and returns home with Jack, making him promise never to tell anyone about his powers, then helps his new friend try to clean up before Dan comes home. Cad shows up at the door with posters of Shoeshine, as Barsinister wants him back for his power, but Jack is able to get him to leave without suspecting anything just before Dan comes home to discover the mess. Meanwhile, Simon and Cad find a new lair beneath the city, then plan a robbery to get the funds they need to rebuild Simon's lab. Cad leads a band of thugs to rob a jewelry store, seen on the news by Shoeshine and Jack, who convinces Shoeshine to use his powers to save the hostages inside. Shoeshine consents, disguising himself with a fish kite so he won't be recognized, and saves the hostages, though Cad escapes with some of the stolen goods. From then on, Shoeshine adopts the alias Underdog, and becomes the city's resident superhero, donning a shrunken red sweater and blue cape as his disguise.
As Underdog enjoys being a hero, Simon's attempts to recreate his serum are unsuccessful time and time again, so he sends Cad to try and obtain a sample of Shoeshine's DNA, hoping to use it to recreate the serum. Shoeshine, meanwhile, is unsuccessful at trying to ask Polly out, but gets an idea when she reveals her crush on Underdog to him, prompting him to ask her on a date as Underdog, which goes successfully. Meanwhile, Cad's attempt to capture Underdog goes unsuccessful, but he does manage to obtain his collar, which has his name and address on it. Simon and Cad go to the house and kidnap Dan, making him believe they have Jack to subdue him, and force him to call for Shoeshine for help. Shoeshine hears Dan's cries, and he and Jack go to rescue him, only for Simon to use both Jack and Dan as hostages to convince Shoeshine to give up his DNA. Not wanting his family to be hurt, Shoeshine complies, and Barsinister makes a whole jar full of pills that contain Underdog's super powers. Simon convinces Shoeshine to take an antidote pill, turning him back into a normal beagle, then feeds the super pills to three German Shepherds he obtained and trained. He and Cad leave the family trapped in the sewers, but thanks to Dan's experience from the police force, he is able to set everyone free and they take off after Barsinister with a hypodermic filled with the super power antidote.
Meanwhile, Simon takes the Mayor hostage in city hall and instructs Cad to attach a bomb rigged with a mind control serum to the roof of the building. Molly and Polly follow Cad after witnessing him head through the service entrance. Shoeshine, meanwhile, smells the bomb Cad is carrying, at first doubting his nose, but decides to trust his instincts and heads inside the capitol building despite being powerless to confront Simon. During a short struggle (in which the mayor is knocked unconscious), Shoeshine accidentally rips open Simon's pocket, gaining access to the super pills and eating one, getting his powers back. He does battle with the three German Shepherds, whom he temporarily incapacitates after hearing Polly and Molly in danger, as they were captured by Cad and tied to the roof of the building beside the bomb. Enraged with Underdog's return to foil his plan, Simon eats one of the super pills and gains super powers himself, fighting Underdog until he distracts him with one of his natural instincts; chasing a Frisbee, which is actually a giant iron shield from a statue. Underdog is knocked down long enough for the German Shepherds to recover, but he convinces them that Simon isn't a good master by pointing out Simon's mistreatment of them (not even giving them real names). The German Shepherds turn on Simon, realizing their role is as man's best friend, not man's best slave, and restrain him with their strength while Shoeshine rushes to disarm the bomb.
The Mayor regains consciousness and stumbles outside, allowing Dan through the police barricade to deal with Simon by injecting the antidote into him. Shoeshine quickly dons his Underdog uniform again and flies to the top of the town hall, freeing Polly and Molly, then taking the bomb to a nearby park, where he digs a hole miles beneath the surface, and buries the bomb. He attempts to rush out of the hole before it detonates, but is caught in the blast halfway out and is blasted so high he reaches orbit, then falls back to the Earth, where he lands unconscious. Jack, Dan, and everyone in the crowd begin to mourn the fallen hero, but Underdog wakes up, singed, but unharmed much to everyone's joy. Dan is reinstated as a police officer, Simon and Cad go to jail, and Shoeshine returns to his normal life protecting the citizens of Capital City as Underdog.
The film closes with the citizens reciting the same phrases from the original cartoon, "Look, up in the Sky! It's a bird, it's a plane, it's a frog! (as one goofy citizen always comments)", to which Underdog replies, "Not bird, nor plane, nor even frog, it's just little old me, Underdog!".
- Jason Lee as Underdog / Shoeshine the Dog
- Peter Dinklage as Dr. Simon BarSinister
- Patrick Warburton as Cad Lackey
- John DiMaggio as Supershep / The Bulldog
- Phil Morris as Maim the Dog ("Supershep #1")
- Michael Massee as Kill the Dog ("Supershep #2")
- Cam Clarke as Attack the Dog ("Supershep #3") / The Little Brown Dog
- Amy Adams as Sweet Polly Purebred the Dog
- Brad Garrett as Riff Raff the Dog
- Jess Harnell as The Astronaut
- James Belushi as Daniel "Dan" Unger
- Alex Neuberger as Thomas "Jack" Unger
- Taylor Momsen as Molly
- John Slattery as The Mayor
- Samantha Bee as Principal Helen Patterson
- Frank L. Ridley as Police Sergeant
- Jay Leno as Himself
- Stephen James Kidd as The Hot Dog Vendor Customer / The Bomb Squad Captain
The bulk of the visual effects work on the film was completed by two London-based companies, Frame Store CFC, and Cinesite. The visual effects supervisor at Cinesite was Ben Shepherd, and the animation supervisor was Alexander Williams.
Underdog was released on August 3, 2007, in 3,013 theaters across the US. It grossed $11,585,121 on its opening weekend, placing in third among that weekend's films in release. As of October 11, 2007, Underdog made $42,937,703 at the domestic box office.
Upon its release, Underdog received largely negative reviews. Rotten Tomatoes' freshness rating for Underdog stood at 15% in March 2010, the lowest of three of director Du Chau's movies at that time (Quest for Camelot and Racing Stripes, receiving 35% and 34% respectively). The movie consensus on that site reads:
"Underdog is a mostly forgettable adaptation that relies far too heavily on recycled material and sloppy production."
Ken Fox of TV Guide said: "If you set your expectations just low enough, or are an easily satisfied 8-year-old, you might have a bit of fun." The New York Daily News saw Underdog as being "made for the Saturday morning TV crowd," claiming that "parents who lead their tykes to this adventure aren't likely to leave disappointed."
Underdog was released on standard DVD and high-definition Blu-ray Disc on December 18, 2007. The DVD and Blu-ray Disc both aspect ratios with Widescreen and Fullscreen.
The DVD shows the first episode of Underdog on the bonus features.
The merchandise for the film was released on July 31, 2007 at the Disney Store. Among the merchandise released that day was a Underdog: Original Soundtrack|soundtrack album from Hollywood Records and books from Disney Publishing Worldwide. A video game was originally going to be released for Xbox 360 and PlayStation 3, but was canceled.
- This is the seventh Disney film to be based on a cartoon TV show that was non-Disney before, the first six being Popeye, George of the Jungle, Mr. Magoo, My Favorite Martian, Inspector Gadget, and The Lone Ranger.