It's another night in Gotham City, and once again the Joker is fleeing the Batmobile in his own car, with Harley Quinn in the driver's seat. When Joker orders her to turn, she protests, but he orders again, and she does so – causing the car to careen down a construction hill. Irritably, he snaps at her to hand him his gun, which she does after rummaging through her bag. He takes gleeful aim at the Batmobile's tires, but the gun turns out to be a dud. Batman gets close enough to snag Joker's rear bumper with a tow cable. As he reverses, Harley presses a button and jettisons the boot, leaving the Batmobile behind.
Back at the hideout, Joker rants about Harley's screw-up with the gun. When Harley timidly mentions that she did get them away from Batman, Joker sarcastically asks if she thinks she's a better crook than him. She dares to say, "maybe," and he tosses her out of the hideout in a rage.
Defiantly, she decides to pull the original heist she planned herself. She goes to the Gotham Museum of Natural History to steal the Harlequin Diamond. She evades the security system with ease, but then the alarms are tripped by Poison Ivy, stealing specimens from the museum's lab. The two women are cornered by the police. Harley grabs one of the specimen bottles and shoots it with her popgun, creating a cloud of gas that disables the police and lets them get away. As they speed away in Ivy's car, she says, "This could be the beginning of a beautiful friendship." Ivy takes Harley to her latest hideout, a house in an abandoned housing development begun over a toxic waste dump. As they bond, Ivy is disgusted at Harley passive acceptance of Joker's abusiveness, and declares that Harley needs to buck up her self-esteem.
For their first official caper, they invade the men's-only Peregrinators Club, tie them up with dropped vine creepers, and then loot the trophy room. Before long, they are committing a spree of crimes all over town, enough to draw Batman's attention. They also become best friends, though Ivy is exasperated at Harley's undying affection for "that psychotic creep."
Elsewhere, Joker is missing Harley in his own fashion: without her, the hideout has become a mess. When he demands why she hasn't come back yet, one of his henchmen nervously shows him the latest headline: "New Queens of Crime." Joker is enraged that Harley is out-doing him as a crook. But he gets his chance when Harley secretly phones him from Ivy's house, allowing him to trace the call. At the same time, Batman analyzes a soil trace from one of their tire tracks, and realizes where they are hiding out. Batman gets there first, but is subdued by one of Ivy's creeper plants. The girls chain him to an old table and dump him into a chemical waste pit. But they re-enter the house to find Joker and his goons, eagerly scooping up their stolen gains. When Ivy protests, Joker gasses her with his boutonnière, but it doesn't work on her. She knocks Joker and his goons to the ground, then drags Harley along with her to make their getaway.
As his goons give chase, they are subdued by Batman, who has managed to escape the trap. Joker picks up a tommy gun and opens fire, despite Batman's warnings about the cocktail of flammable and explosive chemicals they're sitting on. Sure enough, a stray shot ignites a chemical drum, and soon the whole waste dump goes up in flames. Joker's goons flee, while Batman knocks Joker out and then saves them both in the Batmobile.
Speeding away in their car, Ivy exults, "no man can take us prisoner!" Then their car is disabled by a well-aimed shot from Renee Montoya. They are arrested, and sent to Arkham Asylum along with Joker. In Arkham, Joker swears that this is the last time he ever starts a gang with any women in it, while Ivy has some rather resentful feelings towards Harley.