Bill is an alias adopted by Pero, a fugitive from the Cat Kingdom due to his unorthodox friendship with mice. In the lawless frontier of Go Go Town, Pero reinvents himself as cowboy "Bill," using the persona as both disguise and functional role. After the killing of a saloon owner, Bill steps into local conflicts. He allies with the deceased man's daughter, Annie, and a boy named Jimmy, forming the trio referenced in the film's Japanese title as the "three musketeers." Driven by a commitment to justice, Bill takes on the town's criminal elements, including counterfeiters operating under the Big Bad Boss. His strategic efforts unite human allies and a tribe of mouse Indians against these antagonists.

Pero's background informs Bill's adaptability and heroism. Though an outsider, he integrates into the Western setting, applying his traits of courage and loyalty. His actions center on protecting the vulnerable and restoring order to a community lacking legitimate authority.