In the dead of night, a woman runs, panting with fear, through a remote forest, being pursued by a mysterious figure in a jumpsuit. When the woman stops, out of breath, the figure catches up and starts threatening her. The woman calls the figure "evil," and the figure shoots back, "Evil, Mrs. Thomas? I don't control a company that leveled a thousand year-old-forest for a strip mine!" Then the figure sprays Mrs. Thomas with some kind of chemical. There is a creaking sound, and Mrs. Thomas becomes completely petrified.
The next day, at Wayne Manor, Bruce Wayne has an angry phone conversation with a Wayne Enterprises director who made an unauthorized deal with a slash-and-burn operation in the Amazon Rain Forest. ("Shut it down, or you're gone!')
Alfred answers the door to his sometime-girlfriend, artist Maggie Page, whose morning jog has taken her past the Manor. Though pleased to see her, he is a little embarrassed by her open flirting in front of Bruce. While sorting through the mail, Alfred finds a promotional DVD from something called the "Eternal Youth Spa." On it, two pretty young women, Lily and Violet extend Bruce a personalized invitation to a free weekend at the spa. Bruce says he's too busy, but suggests that Alfred and Maggie go in his stead. Alfred stiffly declines, but Maggie is delighted and drags Alfred along with her. ("I've been Shanghaied" he grumbles).
When they arrive at the spa and announce they are there to take Bruce's place, Lily and Violet confer with their boss, hidden in the shadows. The boss tells the girls to let them in – someone has to pay for Wayne's crimes.
The new guests at the spa are treated to a welcome lecture from the spa's founder, Dr. Daphne Demeter. She has invented a new compound, "Demetrite," which has been imbued into the spa's food, drink, water, and air, and has miraculous restorative properties. Alfred does not hide his skepticism, but is pleasantly surprised by the tastiness of the water and fruit samples they are offered.
Bruce is informed that several prominent industrialists, including Mrs. Thomas, have disappeared mysteriously. In her apartment, Batman finds another promotional DVD for the spa. He calls Alfred at the spa, who duly inquires about Mrs. Thomas and is told she left unexpectedly a few weeks earlier. Alfred adds that he is having a wonderful time, and will return to the Manor a new man.
Indeed, when Maggie drops Alfred off after their weekend, he bids her a fond goodbye, complete with a kiss on the cheek. Later that day, Batman pulls the Batmobile into the Batcave and is taken aback to see Alfred decorating the cave with potted plants. Now a firm believer in Demetrite, Alfred takes a dose in a glass of water, and suggests that Bruce do the same. However, he suddenly sways, feeling faint, and Bruce has to help him upstairs into bed. Suspicious, he takes a sample of the Demetrite for analysis.
The next morning, Alfred rises from his bed, takes in the morning sunshine, and instantly feels better. He sees Maggie outside his window, and they agree that they are sharing the same thought – they want to return to the spa. They do so, telling Dr. Demeter about what a difference she has made to them. With a sinister smile, Demeter motions Lily and Violet forward, who spray Alfred and Maggie with chemicals, petrifying them.
In the cave, Batman tests the demitrite by mixing it with the plasma found in the human bloodstream. It causes a violent chemical reaction, and an immense vine immediately grows out of the test tube. Batman runs upstairs and finds Alfred gone, and takes off for the spa.
Entering the spa's cathedral-size greenhouse at night, Batman is surprised and taken prisoner by Violet and Lily. Then "Demeter" reveals herself as Poison Ivy. The Demetrite, she reveals, is a special growth enzyme extracted from a rare South American tree she has cultivated. With it, she has produced her "collection" – her victims, all of whom own companies that have committed some environmental crime, and all of whom have now been transformed into humanoid trees. After several more sprayings, the transformations will be permanent. She claims to be after Bruce Wayne for his part in the slash-and-burn operation (not knowing it was actually one of Bruce's directors), but for now is content with his friends. She shows him Alfred and Maggie among the trees, enraging him.
Ivy says that she and Batman should be on the same side – they both punish evildoers. Batman tells her what he thinks of that, and she orders Violet and Lily to spray him with a triple-strength mixture that should transform him almost instantly. But it doesn't work, and Batman advances, explaining that he analyzed the Demetrite and coated his suit with a tailor-made herbicide. Violet and Lily back away, still spraying, but Batman subdues both of them. Ivy somersaults through the branches, firing her crossbow at Batman. One of her arrows misses and shatters a flask in her laboratory, spilling the enzyme onto the earth floor of the greenhouse. Instantly, an enormous tree grows out of the ground at an accelerated rate, scooping up Ivy and her henchwomen in its expanding branches and ramming a hole in the greenhouse ceiling.
Ivy's victims are treated, and Alfred and Maggie share some rest time at the hospital. Bruce enters with a flowerpot to brighten their room, and both Alfred and Maggie recoil.