Role in the film
Polly was first introduced as Molly's pet dog, she first met Shoeshine (who later became Underdog). at the park where she admitted that he was a cute beagle but expressed that she wanted to be in a relationship with a dog who could sweep her off her feet, they both agreed to be friends however and Polly and Molly left the park shortly after.
Polly and Molly were ambushed by a pair of thugs who were trying to steal Molly's backpack, Shoeshine heard Molly's cries for help and Polly's whimpering and (getting used to his newfound superpowers). was able to subdue the two thugs and save Molly and Polly. Polly however, didn't realize that it was Shoeshine that rescued her and her owner.
Polly then became interested in Underdog when she heard about him on the news about thwarting a robbery that occurred at a jewelry store, weeks later when she met Shoeshine at the Library she expressed her interest in wanting to meet Underdog but she didn't know that Shoeshine was really Underdog. Shoeshine then pointed out that Underdog would usually respond a any call for help. Polly then called Underdog to her aid that night making up the excuse that she was out of dog food. The two of them then went out on a date and had spaghetti and meatballs while sitting atop the roof of the Capitol City building.
The Next day, Polly told Shoeshine about her date with Underdog, this was cut short as Shoeshine and his owner Jack Unger, had to leave as Jack's father was captured by Simon Barsinister (whom was the reason the Shoeshine gained the superpowers and became Underdog).
Later Molly and Polly found a gathering outside the city hall as Barsinister had taken the mayor hostage. Molly then spotted Barsinister's henchman Cad enter the building through the service entrance and she and Polly followed him up to the roof. There, they found Cad strapping a Bomb to the roof of the building, Polly's barking gave her and Molly away and Cad tied them to the roof of the building beside the bomb. Underdog showed up and freed them and Polly and Molly took the DNA cocktail to the police under Underdog's instructions.
The following Day, Polly told Shoeshine about Underdog saving the town and it's citizens from falling under Barsinister's control. this was cut short again as Shoeshine had to leave because of another call for help.
- The scene where Underdog and Sweet Polly Purebred eat spaghetti together is a parody of the similar iconic scene from Disney's Lady and the Tramp.