Batman Beyond

While Terry is out walking Ace, Bruce Wayne's dog, Ace savagely attacks a suspicious-looking man on the street and then runs away. When Bruce traces the serial number on the man's gun, he discovers the man is none other than Ronny Boxer, a longtime purveyor of illegal sports, including dog sports. Boxer is also Ace's former owner; Bruce took him in when Boxer's dog-fighting ring was busted by the police. While Terry goes undercover to get close to Boxer, Bruce looks for Ace, all the while wondering exactly what Boxer did to Ace in the past.