A giant creature climbs his way up a building and enters an apartment through the window. It seems to know the territory well as it moves towards an abstract painting, tearing it away. A safe is revealed and the strange being wastes no time disposing of the door and gathering its contents. The creature is interrupted by the owner, but is quick to knock him out-cold and continue its way. The man loses consciousness while the creature's shadow recedes, exiting the apartment. At the Police Department, Commissioner Gordon and a police scientist analyze a strange green substance that turns out to be a poison. The same poison was used four different times during the past month. The scientist admits to not identifying it and manifests his worry that he might not be able to keep the victims of the poison alive for much longer. The doctor's statement determines Batman to reveal himself from behind the shadows. He proceeds in pointing out his suspicion of Poison Ivy in the matter, but the Commissioner insists that she is not a valid suspect and admits his belief that Poison Ivy has turned a new leaf. He reveals that she had been released from Arkham Asylum after apparently completing her therapy successfully. She also had married her psychiatrist, Steven Carlyle, becoming the step-mother of his two children, Chris and Kelly.
The next day, Batman and Commissioner Gordon pay a visit at the Carlyle residence and express their suspicion regarding the mysterious robberies and poisoning attacks. Ivy sustains her innocence and the veracity of her redemption, explaining her attachment to her new family and to her new life. She mentions her inability to have children of her own as a result of her hyper-immune system. She insists on convincing the men that she is happy with her husband and children accepting her for who she is, that she would never commit crimes after finding their affection, claiming that Poison Ivy "is dead".
At Gotham University's dorms, Dick Grayson is flirting with a girl named Cindy. He is interrupted by a call from Batman, requesting him to find information on Steven Carlyle. The girl leaves and Dick is left alone in the room. He notices a shadow near the door and moves towards it, hoping it would be Cindy, but instead he is greeted by the strange creature of the previous days. It smashes through the door, knocking off a nearby lamp. Unable to see his attacker, Dick receives a direct blow. Police investigate the crime scene; some searching for clues, others comforting Cindy, apparently the only witness. Bruce and Commissioner Gordon are also there questioning Cindy. The Commissioner tells Bruce about his theory that Dick had been kidnapped in order to blackmail him. Bruce considers and decides to leave the crime scene. He gets in his car and leaves the area. While driving, he is ambushed from behind by the strange clawed creature. Struggling to keep the car on the road, Bruce asks the creature what it is after. It demands 55 million dollars be sent to him at the docks, at midnight, in exchange for Dick's life. Bruce returns to his manor, prepares the money and decides to check on Poison Ivy. He follows her throughout the day, but does not find any incriminating evidence. Bruce shows up at the docks with the suitcase containing the 5 million dollar ransom and waits for a sign from the kidnapper. Shortly after, his name is called out by a voice hidden behind a row of crates. The creature demands for the suitcase, but Bruce remains unfazed and requests to see his ward. The creature extends his arm and shows a tied-up and blindfolded Dick struggling to release himself, then hides again, its voice growing angrier while asking again for the money. Bruce kicks the suitcase towards the crates and asks that the boy be sent out. Instead, the creature attacks him head on and we finally see its body — a giant green monster. Bruce only dodges the creature's blows, but in the end falls over, into the water. The monster turns and grabs a hold of Dick, preparing to throw him into the water as well. Before he can proceed, Batman rises from the water and cuts the monster's claws with his Batarang. The creature panics and Batman takes that opportunity to strike him frontally. The monster manages to escape by throwing Batman into a row of crates and running away, abandoning the docks. Afterwards, Batman and Robin check notes in the Batmobile. Bruce tells Robin that Ivy is no longer his suspect because "all she wants is to be left alone with her husband and sons". Robin shows his surprise when hearing about Steven Carlyle's 'sons' and tells Batman that in truth, Carlyle doesn't have custody of his kids and that Chris and Kelly are in fact girls. And with that, Batman and Robin return to investigate the Carlyle house. Robin checks the house and notices everyone is inside, asleep, except Ivy. Batman takes note and tinkers with the garden's pipelines before perusing it. The two look around and discover a hidden laboratory. After closer inspection, they find Dr. Carlyle in a testing tub. Shortly after freeing him, Batman notices a pack of plants moving. Closing in on them he is shocked to see babies emerging from the thicket. Caught off guard, Batman, Robin and Dr. Carlyle are captured by Poison Ivy's vines. While Ivy confesses her real intentions and explains her experiments, she feeds the babies a growth formula, turning them into monsters and demanding that they kill the three men.
Meanwhile, Batman manages to cut himself and Robin free and fight the creatures. Batman stuns them by plucking nearby cables and electrocuting them. They then take Carlyle and try to run away only to find more mutants waiting for them. Although outnumbered, the Dynamic Duo manages to come through it all. Slowly, Batman draws them into his trap. When the creatures were in appropriate position, he turned on the herbicide and finished off his attackers.
Seeing her plan fail, Ivy shows herself and questions Batman about where her plan had flawed. Batman answers while closing in on her; Ivy maintains her position and reveals her escape plan before collapsing and decaying. She was only an artificial copy, just like her experiments. The police arrive soon after to wrap up the case and use the laboratory's contents to form an antidote for Ivy's poison.
Soon afterwards, the real Poison Ivy flies away from Gotham. Inside the plane, she skims through her photo album, fixating her gaze over her wedding photo, shedding tears upon it.