In Sparta Prison, Batman and Commissioner Gordon try to talk a prisoner named Dubek into talking about his former employer the Joker or risk staying in solitary confinement. Dubek refuses, but the Joker, knowing that eventually his former stooge may crack, decides to put a plan in motion to silence him. When Ted Grant puts on a boxing match for the prison, Dubek is picked to represent the prison in the fight. The Joker puts a rare tropical disease into Dubek's water bucket, which also infects the other prisoners. A doctor gives them 72 hours to live.
By order of Police Commissioner Gordon, Michael Dubcek, a former stooge of the Joker's, is left to rot in solitary confinement, at Sparta Prison. Gordon hopes that sweating it out in solitary will eventually loosen Dubcek's tongue, regarding the Joker's criminal operations. After a week, Dubcek is close to cracking. Fearing that Dubcek may talk, the Joker makes plans to silence his former henchman. At Hillman's Gym, ex- heavy weight champion of the world, Ted Grant, trains for an exhibition match at Sparta Prison. Grant takes on a retired bantamweight fighter, Whitey Ross, as his ringside assistant. Against Gordon's wishes, the warden of Sparta Prison releases Dubcek from solitary, to face Grant in the ring. A simple exhibition fight becomes a brutal grudge match, but by the fourth round, Grant lays out Dubcek on the canvas. Two days later, every inmate at Sparta Prison is suffering from a rare tropical fever. When it's revealed that Dubcek was the first man to come down with the disease, the warden confesses to releasing Dubcek from solitary to box Grant, then allowing him to dine with the other inmates after the match. The infection is traced to Dubcek's water bucket, which had been switched by Grant's man, Ross. In truth, Ross was really the Joker's man. To silence Dubcek, the Joker has imperiled the lives of over 600 convicts.
Fortunately, a lone scientist, at the Barston Institute, has been researching the rare disease inflicted upon Sparta Prison's inmates. This scientist has one laboratory animal, a dog called "Spot", whose blood is teeming with enough antibodies to cure the prison populace. Batman and Wildcat provide an escort for the armored car carrying the invaluable canine to the airport. Thanks to a wiretap on Gordon's personal line, the Joker has been tipped off to his enemies plans. Driving ahead of the armored car, the Joker releases his deadly laughing gas through his vehicle's exhaust. The armored car begins weaving wildly into traffic, before crashing into a pylon. With the guards incapacitated by the Joker's gas, the Clown Prince of Crime takes possession of Spot. Wildcat takes off in pursuit of the Joker's car. Spot leaps out the window to chase another dog. Wildcat breaks off his pursuit of the Joker, to trail Spot. The chase takes Wildcat across an opening drawbridge. The two dogs leap onto a passing barge, while Wildcat is forced to vault the bridge. Wildcat, Batman, the Joker, and Gotham City's police are all on the hunt for Spot. After being put off the barge, Spot ends up with a young boy, who spends several hours playing with the dog. Spotted by the Batman, the boy runs away, as does Spot. Spot is next found by a vagrant who sees the dog as his ticket to the big time. Ultimately, Spot is picked up by local dogcatchers. At the dog pound, Spot is claimed by the Joker.
Breaking into the Batman's secret police frequency, the Joker instructs Batman and Wildcat to come to Jefferson Square Garden, if they ever want to see Spot alive again. Holding a gun to the dog's head, the Joker forces Batman and Wildcat to fight for his entertainment. The two heroes are fitted with spiked Roman cestus gloves, to ensure a particularly bloody match. Batman and Wildcat brutally lay into one another, trading bone crushing blows with each other. The punishing battle goes on for several moments, until a final haymaker from each combatant drops them both to the mat. With Batman and Wildcat reeling, the Joker taunts the two heroes with the knowledge that the cestus gloves were laced with the same rare fever germs plaguing the Sparta Prison inmates. Once the Joker kills the dog, everyone infected will die. Suddenly, Spot bites the Joker. Enraged, the Joker commands his henchman to dispose of the dog. Batman quickly convinces the Joker into believing that Spot's bite carried the fever germs, too, and that the Joker is infected. Sensing a bluff, but not willing to take the risk, the Joker rushes to retrieve the dog. Tied to a rock, Spot is dropped into the river. The Joker leaps in after the dog, forgetting he doesn't know how to swim. Spot keeps the Joker's head above water long enough for Batman to rescue them both. The Sparta Prison populace, along with Batman and Wildcat, all receive inoculations of Spot's anti-bodies, while the Joker is turned over to the proper authorities.