In Gotham City, the iconic duo, Batman and Robin (Chris O'Donnell), are on a mission to thwart the villainous plans of Mr. Freeze, who aims to seize a stash of diamonds. In a thrilling confrontation, Mr. Freeze manages to freeze Robin, leaving Batman with a mere 11 minutes to rescue his sidekick. As they delve deeper, they uncover Mr. Freeze's past—he was once the esteemed scientist Victor Fries. Following a tragic accident in his cryogenics lab, Victor became reliant on a diamond-powered subzero suit to sustain himself, all in a desperate search for a cure for his beloved wife, Nora, who is afflicted by the terminal MacGregor's Syndrome.
Hauntingly, Nora is now preserved in cryogenic sleep, with Victor's ultimate goal to freeze Gotham City and extract billions to fund his research for her cure. Meanwhile, Dr. Pamela Isley, an ambitious botanist, experiments with a potent serum called "Venom", aiming to create formidable mutant plants to combat humanity's destruction of nature. However, she soon finds her work exploited by her senior colleague, Dr. Jason Woodrue, who transforms a small-time prisoner into the super-soldier Bane. After a violent confrontation, Pamela emerges as the enchanting Poison Ivy, bent on achieving botanical supremacy and exacting revenge on Woodrue with a deadly kiss.
As events unfold, Alfred Pennyworth's niece, Barbara Wilson (Alicia Silverstone), arrives from England, seeking refuge at Wayne Manor as she prepares for university. Amidst the chaos, Alfred reveals that he too suffers from MacGregor's Syndrome, adding to Bruce Wayne's urgency. Poison Ivy, accompanied by Bane, makes her disruptive entrance at a Wayne Enterprises conference showcasing a revolutionary telescope, challenging Bruce to prioritize environmental protection, to which he firmly declines.
At a Gotham Botanical Gardens fundraiser, Ivy unleashes her pheromone powers, captivating attendees—including the Dynamic Duo—before Mr. Freeze crashes the event. Although temporarily apprehended by Batman and confined to Arkham Asylum, Freeze is soon liberated by Ivy. Under Ivy's manipulation, Freeze believes Batman is responsible for Nora's distress. Together, they plot to transform the observatory's telescope into a freezing weapon, endangering the entire planet.
As tensions rise, Robin, infatuated with Poison Ivy, strays from Batman's side, only to find himself at her mercy. Yet, he cleverly avoids her lethal kiss with Batman's foresight in mind. In an intense showdown, Batgirl—a new ally—arrives just in time to assist, ultimately subduing Ivy.
In a climactic finale at the Observatory, amidst Bane's violent attack, the heroes unite to save Gotham by recalibrating the telescope to reflect sunlight, thawing the city from Freeze's icy grip. Resolutely, Batman presents evidence of Ivy's betrayal to Freeze, revealing that he has healed Nora, assuring him that he can continue his research at Arkham. Batman's compassion shines through as he takes possession of the cure for Alfred, thus addressing multiple layers of trauma caused by the villains.
In the aftermath, Ivy finds herself caged with Freeze, who vows revenge, while at Wayne Manor, a restored Alfred welcomes Barbara as they prepare for a brighter future in the fight against crime. With alliances strengthened and new heroes emerging, Gotham's destiny hangs in the balance.
