Batman is staking out S & D Pharmaceuticals. Suddenly, eerie laughter fills the air, as Batman is attack from behind. Batman's assailant proves both faster and stronger than the Dark Knight Detective. Wrenching the man's trench coat off, Batman's adversary is revealed to be the Creeper. Batman ceases the fight, and offers his hand in friendship. The Creeper immediately kicks Batman off the roof. Batman is saved only by grabbing the bat-rope, still in place from when he scaled the building. Batman tends to his injuries at the Wayne Penthouse.
Batman's investigation into the fire bombing of several pharmaceutical companies has turned up one commonality, monofragilic acid. Batman suspects the bombings have been done to cover up the theft of large quantities of the chemical. The Creeper returns to the laboratory of Ishmael Yatz. Though he has failed to obtain anymore monofragilic acid, the Creeper reports that he's done away with Batman. The Creeper's mood swings from psychotic glee to heartfelt grief and remorse. He pleads with Yatz to find a cure for his condition. Yatz assures the Creeper that he is getting close to a solution.
Inexplicably, Yatz orders his assistant to attack the Creeper. Yatz states that he was merely testing the Creeper's reflexes. As the Creeper exits, Yatz confides in his assistant that the cure he will be giving the Creeper is the cure of the grave. Researching monofragilic acid, Batman learns that it can be used to modify human physicality, giving the subject enhanced strength, speed, and agility. Batman recalls that the Creeper told him that a Doctor Yatz injected him with the formula that gave him his enhanced abilities.
Yatz was murdered by foreign agents, but his son, Ishmael, maintains a laboratory on the outskirts of Gotham City. Batman dons a disguise to gain entry into Yatz's laboratory. Yatz gives the Creeper the antidote to his father's formula. In reality, though, Yatz has given the Creeper poison. The Creeper lashes out at Yatz, only to discover that Yatz has duplicated his father's formula, and given himself the same abilities the Creeper possesses. Yatz assistant moves in for the kill, but is stopped by Batman. The Dark Knight hurls the man onto a table covered with various chemical beakers.
The lab fills up with noxious smoke, generated by the intermingling chemicals. By the time Batman clears the air, Yatz and the Creeper are gone. Batman pursues Yatz to the Fantino Narrows Bridge. There, Yatz is meeting with foreign buyers for his father's formula. Out of the darkness, the Creeper attacks Yatz. Batman takes down the foreign agents. With his dying breath, the Creeper sets about to murdering Yatz. Batman slams into the two men, separating them, and rendering them both unconscious. Suddenly the Creeper's features begin to blur, until he, once again, becomes reporter Jack Ryder. The poison, mixed with Yatz' formula already in his system, has returned Ryder to normal.
All of Gotham City has turned out for the gala premiere of the new movie, "The Stepfather". In honor of the film's setting, Gothamites have come to the premiere in period attire. Among the attendees are Police Commissioner James Gordon, his daughter Barbara, and private investigator, Jason Bard. Also in attendance are Floyd Marcus, and his bodyguard Larry "The Blimp" Cooper. The film is reputedly based on Marcus' underworld career as the former Kingpin of crime in Gotham City. Marcus dismisses any similarity the film has to his own life. Suddenly, a vintage car whips around the corner, guns blazing from the windows. Cooper shields Marcus from the gunfire, only to learn that the attack was a publicity stunt. A second vintage car trails after the first.
This time the attack is no publicity stunt. Marcus is shot down in the street. Gordon rushes to his car to pursue, only to learn that his daughter has already taken it. Gordon realizes that his daughter is pursuing the killers, in her guise as Batgirl. Barbara recognized Marcus' killer as Mike Marcus, his stepson. Barbara travels to Marcus' estate, then changes into her Batgirl costume. Batgirl searches Marcus' garage and discovers that one of his vintage cars is missing. Soon, a moving van arrives, and unloads the car in question. One of Marcus' men notices Batgirl's wet boot impressions outside the garage. Marcus pulls into the garage, then sends his men away. It is then that Batgirl realizes that Marcus did not kill his stepfather.
Mister Hammond has rather foolishly invested every last penny he has in a series of fish tanks. His hope is that a local aquarium will purchase the tanks from him. The superintendent, Finley, though, refuses Hammond. Faced with financial ruin, Hammond lingers as Finley feeds the piranha, for a crowd of onlookers. When Finley spies a knife at the bottom of the piranha tank, he immediately fires the caretaker, Brady. Hammond sees Brady's public firing as an opportunity. Fishing the knife out of the piranha tank, Hammond uses it to stab Finley to death.
Hammond then contacts the police to report the murder. Hammond pins the crime on the caretaker. Detective Arvin Steinmetz, though, immediately sees the hole in Hammond's story. Hammond claims he saw Brady reach into the piranha tank and grab the knife to kill Finley. Steinmetz points out that Brady, in his role as caretaker, would have known better than to stick his bare arm into a piranha tank. Catching Hammond in an obvious lie, Steinmetz accuses Hammond of being the killer. Hammond confesses his guilt, and is tried and sentenced for the crime.
This story was originally published in Dale Evans Comics #1 (September-October, 1948). A plot summary for this story can be found on that issue page.