Plot

Bruce Wayne / Batman (George Clooney) and Dick Grayson / Robin (Chris O'Donnell) attempt to stop Mr. Freeze (Arnold Schwarzenegger) from stealing a cache of diamonds. During a confrontation at the natural history museum, Freeze steals a large diamond and flees in a spaceship. Freeze travels with an entourage of his minions who attack Batman and Robin on ice skates, suing the ice created by Freeze during his assault.

Batman is captured inside the spaceship while Robin tries to climb into it as the spaceship is in flight. Freeze tells Batman that the spaceship will freeze at an altitude of 30,000 feet where his heart will stop working and then jumps out of the craft wearing a sort of butterfly wings. Robin breaks into the craft to save Batman. Batman and Robin destroy the craft before jumping out pursuing Freeze on spacecraft panels that they use as wind surfing boards. Freeze jumps into a furnace after he freezes it on his way down and is followed by Batman and Robin. Freeze then uses his freeze gun on Robin, leaving Batman unable to pursue.

The super-criminal notifies Batman that he has 11 minutes to thaw his partner. They learn that Freeze was once scientist Victor Fries, who became dependent on a diamond-powered subzero suit following an accident in a cryogenics lab he was occupying to find a cure for his wife, Nora (Vendela Kirsebom), who was suffering from a terminal illness called MacGregor's Syndrome.

Nora has now been Cryo frozen by Victor, who is hoping to continue his research to develop a cure for her disease. Victor plans to freeze the entire Gotham city so as to demand the billions of dollars required to complete his research.

At a Wayne Enterprises lab in Brazil, botanist Dr. Pamela Isley (Uma Thurman) is experimenting with the strength serum "Venom" to create mutant plants capable of fighting back against mankind's destruction of flora. To her anger, she discovers that her senior colleague Dr. Jason Woodrue (John Glover) used her Venom samples to transform a diminutive prisoner into the "super soldier" Bane. When she refuses to become the mad scientist's partner, Woodrue kills her with animal-plant toxins and chemicals. Instead, Isley is mutated by the toxins into Poison Ivy, who kills Woodrue with a poisonous kiss, destroys the lab and escapes to Gotham City with Bane, concocting a plot to use Wayne's money to support her research.

Alfred Pennyworth's (Michael Gough) niece Barbara Wilson (Alicia Silverstone) makes a surprise visit from England and is invited to stay at Wayne Manor until she returns to university. Alfred is revealed to be suffering from MacGregor's Syndrome.

Ivy arrives in Gotham City with Bane ( Robert Swenson) as her henchman. She interrupts a Wayne Enterprises press conference at the Gotham Observatory where a giant new telescope is being unveiled, exhorting Bruce Wayne to implement a proposal that would safeguard the natural environment at the expense of millions of human lives, which Bruce summarily declines. Batman and Robin decide to lure Freeze out using the Wayne Family diamonds and present them at a Wayne Enterprises charity event.

Ivy later appears at the Gotham Botanical Gardens fundraiser, seducing everyone present with her pheromone dust, including the Dynamic Duo, who are there to protect a diamond on auction from Mr. Freeze. When Freeze crashes the event as expected, Ivy is instantly captivated by his "ruthless charm". Despite being captured by Batman and detained at the Arkham Asylum, Mr. Freeze is released by Ivy.

Dick discovers that Barbara has been participating in drag races to raise money for Alfred, who is dying of MacGregor's syndrome; a fact he kept from Bruce and Dick, but his niece is secretly aware of his situation and is trying to find treatment for him.

Batman, Robin, and the police arrive at Freeze's lair in response to his escape, discovering Nora preserved in a cryogenic chamber and that Freeze has developed a cure for the early stages of MacGregor's syndrome. The villains soon secretly arrive to recover Freeze's diamonds and Nora. Ivy then proceeds to turn off Nora Fries' life support, steals the diamonds, and seduces Robin, escalating tensions between him and Batman. Ivy makes Mr. Freeze believe Batman did it, thereby persuading him that they should kill Batman as well as the society that created him. At her hideout, Ivy convinces Freeze that Batman was responsible for her attempt at Nora's life and he then resolves to make humanity suffer for revenge, with Ivy plotting to repopulate Earth using her mutant plants afterward. Freeze and Bane commandeer Gotham Observatory and convert the new telescope into a giant freeze ray, while Ivy uses the Bat-Signal to contact Robin.

Barbara discovers the Batcave, where an artificial intelligence version of Alfred reveals he has made a suit for Barbara. Barbara dons the suit and becomes Batgirl.

Meanwhile, Robin has become infatuated with Ivy and rebels against Batman, just as the villainies planned. He eventually goes to meet her at her garden hide-out. Ivy's venomous kiss fails to kill Robin because Batman had prevailed on him to coat his lips with rubber. She tries to drown Robin in her lily pond and entangles Batman in her crushing vines, although they are able to free themselves when Batgirl arrives and traps Ivy in her own floral throne.

When the Trio arrive at the Observatory to stop Freeze, the criminal has already frozen Gotham. Bane attacks Robin and Batgirl, but they incapacitate him and restore him to his original human state. The two younger heroes save Gotham by re-positioning the observatory's satellites, using their mirrors to reflect sunlight from outer space to thaw the city.

Freeze accuses Batman of taking Nora's life, when Batman shows Freeze video footage of Ivy's confession about pulling the plug on Nora, but reveals that he himself restored Nora. He vows that Freeze will be allowed to continue his research at Arkham Asylum to heal Nora. Batman asks Freeze for the cure he created for the first stage of MacGregor's Syndrome for Alfred and Freeze atones for his misdeeds by giving him two vials of the medicine he had developed.

At Arkham, Ivy is joined in her cell by Freeze, who vows to exact revenge on her. Back at Wayne Manor, Alfred is cured and Bruce invites Barbara to live with them and continue helping him and Dick fight crime as Batgirl.