Terry takes Ace for a walk as Bruce visits the chiropractor. Suddenly, Ace starts running out of control, slipping his leash and chasing after a man on the street, who jumps into a car and flees. Checking Terry's description against the files identifies the man as Ronny Boxer, a dog fight promoter.
As Terry investigates, pretending to be a kid with a fighting dog to sell, Bruce gazes at Ace's dishes and bone, wondering what's happened to him.
A flashback reveals Ace's story. He was sold to Boxer by a corrupt dog pound attendant and raised as a fighting dog. However, the police raided the dog-fighting ring, and Ace escaped in the confusion. Living as a stray, he wandered into Crime Alley just as Bruce paid his annual visit to the scene of his parents' murder. One of the local Jokerz attempted to mug Bruce, but Ace pounced on him. Bruce angrily sent the mugger packing, and then saw the injured dog and brought him home.
In the present, Terry's gambit has given him a lead on Boxer's hideout. Paying a visit as Batman, he discovers a chem lab and containers of serostone growth hormone. Not only is Boxer forcing dogs to fight, he's dosing them to make them tougher and meaner. Meanwhile, Ace is found by the corrupt attendant, who removes his tags and sells him to Boxer, who recognizes him from before.
Batman tracks the dog-fighting operation to a cargo ship in the harbor. Boxer sets a trap for him, pitting him against an elephant-sized dog, the subject of his first growth hormone experiments. The beast slams into the walls of the arena, knocking Ace's cage down some stairs and loosening its door. Ace escapes and chases Boxer, then returns just as the creature swallows Batman. Ace startles the beast, enabling Batman to escape and defeat it. Batman then collects Ronny Boxer, who is helplessly hanging from the stern of the ship.
The episode closes with Terry bringing Ace home to Bruce, who greets him with obvious relief.
- Attack of the 50-Foot Whatever: Boxer unleashes a huge hormone-mutated dog on Batman in the final showdown.
- Beastly Bloodsports: Boxer's dog-fighting operation. It's clear that Bruce and Terry find him disgusting even by criminal standards, especially when they find out about the growth hormone.
- Call-Back: Serostone is the stuff that transformed Charlie Bigelow into a hulking brute in "Big Time."
- Dark and Troubled Past: Ace was 'adopted' by Boxer and cruelly brought up as a fighting dog. He managed to escape the fighting ring, but not his training as a fighting dog.
- The Dog Bites Back: Ace recognizes and chases Boxer, bringing him to Batman's attention and putting an end to his dog-fighting operation.
- Grave-Marking Scene: Even in his old age, Bruce continues to visit the scene of his parents' murder in Crime Alley.
- Missed Him by That Much: Ace and Terry almost find each other twice—once, Ace spots and barks at the Batmobile but Terry fails to notice; later, Ace wanders by just as Terry leaves, picks up his scent, but then gets captured by the corrupt dog pound attendant.
- Not So Stoic: Bruce told Terry that Ace was enough of a survivor to be okay on the streets until they found him again. The scene where he stared at Ace's dishes and his obvious relief when Ace comes home show that he was more worried than he'd let on.
- Twisted Christmas: Bruce's visit to Crime Alley takes place during the Christmas season, meaning that his parents' murder (and his subsequent anniversary visits to the scene) took place at this time of year.