The figure of a limp woman with long curly hair is seen drifting underwater, and a voiceover questions the end of a life that never truly existed. This life ended in murder, the victim herself unsure of who her killer was and why she was killed. Her death was only metaphorical, for in her place a new life begins.
The story starts with Patience Phillips (Halle Berry), a timid woman who can't stand up for herself. She lets people walk all over her, and has wasted her artistic talent working for a cosmetic company as a graphic designer. George Hedare (Lambert Wilson) runs the company with his supermodel wife, Laurel (Sharon Stone). Laurel knows that Patience is capable, but seems mostly indifferent, only using her in own power struggles with her husband. George, an abusive boss in general, finds plenty of opportunity to mock Laurel.
The company is about to launch BeauLine, a new anti-aging product that Laurel had secretly been using for years. The advertisement is designed by Patience but George, unsatisfied, demands that she redesigns it. Upset and frustrated, Patience returns home and sees a cat looking up at her. Shaking her head, she goes to sleep, but her loud neighbours keeps her up and she is unable to rest. Next morning, the cat is on her window ledge. When she approaches, she realizes the cat has climbed up to a higher ledge and was potentially stuck. To try and reach the cat, Patience climbs out her window ledge, and a passing detective mistakens her intentions as suicidal. He ends up rushing into her apartment and pulling her back in. Patience would have thanked him but realizes she is late for work, in her hurry to leave she drops her wallet. The detective, Tom Lone (Benjamin Bratt), tracks her down to her workplace and returns it to her, and both agree to a coffee date.
Later that night, Patience goes to the production factory to deliver her artwork, and she accidentally hears that the BeauLine has unsafe health problems. Stunned, she backs into several tools, revealling her presence. Two men chase her into a large waste water pipe, where she is trapped, and they flush her into the river, where she drowns. The cat that has been appearing in front of Patience appears by her body and gathers several other cats around her.
Patience eventually drags herself unsteadily up and returns home. Her movements are off and uncontrollable, and she ends up breaking the glass through her own window. When her neighbours start their usual loud party again, she yells at them to quiet but they again ignore her. The old Patience would have backed down, but now she strodes down, knocks on their door, and promptly beats up the host and destroys the music equipment in front of many shocked guests. When she wakes up the next morning, she almost dismisses everything as a dream until she sees the broken glass. Unfortunately, her memories of what happened before that are blank, and so she cluelessly returns back to work.
At work, George loudly yells at Patience in front of all her coworkers for not handling in her project. She tunes him out, which angers him further, and when she apologizes he only continues. Patience than apologizes for wasting her time in the company, and is fired, but as soon as George leaves her colleagues swarm around her in congratulations. Patience packs up her things and while she and her friends are walking home, two dogs instantly begin barking at her and she instinctively hisses at them. When questioned, she explains her behaviour as allergies. When they pass by a jewelry store, Patience's attention is caught by a beautiful collar-like necklace, but is reminded of her new unemployed status and leaves longingly. Once home, she sees the same cat again, and finally close enough she was able to find the address on the cat's owner.
The cat belonged to an older woman named Ophelia, a former professor who lived alone in a house full of cats.
She explains that the cat constantly appearing in front of Patience is named Midnight, a rare Egyptian Mau who likely has been watching Patience for several days, knowing that she was about to die. The cat has special powers that are given only to a few deserving women after they die. Hence after Patience died, she is reborn with new catlike powers. History will tell of women with the senses and agility of a cat, and Patience has now joined their ranks. Patience and Catwoman are two completely different people, yet they are one.
Now on the prowl, Patience digs up a leather outfit she once received as a gift but never wore, and donned on the cat mask Ophelia gives her. Catwoman finds the man who had tried to shoot before drowning Patience, but after a fight she learns he had no idea who the girl he killed even was or exactly why he had been suddenly called in to eliminate her. He only knows it was something to do with the company, perhaps some sort of corporate scandal, and that she should probably check with the people at the top. Catwoman agrees and goes to corporate headquarters, only to find another man she had seen on the night of her death lying dead on the floor. A passing janitor walks into the scene and mistakens Catwoman as the man's murderer, she is forced to flee when the police are summoned.
Now a wanted criminal to the public, Catwoman sneaks into George's private residence, where she briefly gets into a fight with Laurel. Catwoman demands to know where George is, telling Laurel about the terrible effects of BeauLine. Laurel gives Catwoman a hotel address as well as a cell phone so the two of them could keep in contact. Catwoman finds George with Drina, another upcoming model, in a private balcony watching a dance. Unfortunately, a security officer had already spotted Catwoman and called the police. When Drina briefly leaves and Catwoman tries to interrogate an uncooperative and still abusive George, she is cut short by the police suddenly appearing. Left with no choice, she jumps off the balcony and onto the stage before briefly dropping down below. The audience mistakens her brief appearance as part of the show and loudly applaud while below, Catwoman finds Tom with a gun. The two of them fight while she flirts and even kisses him before reinforcements arrive and Catwoman is again forced to flee.
On their date, Tom tells Patience about his encounter with Catwoman. She doesn't seem jealous, and the two continue having a great time until Tom finds a diamond-studded claw in Patience's home, something he instantly recognizes as part of Catwoman. He takes a cup Patience had drank from to the police lab, where analysis prove Patience and Catwoman as the same person. Meanwhile, Patience wakes up alone in the bed, and receives a call from Laurel who says she found evidence of her husband's crimes. Catwoman goes to the Hedare residence only to fall for a trap - George had been clawed at and fatally shot, and Laurel tosses the gun into Catwoman's hands so that the police would identify her as the killer.
Her agility and Laurel's exercise outfit helps Patience escape the police and allows her to return home, but Tom is waiting for her. He knows if she wanted to she could quickly disarm him, but she doesn't and lets him arrest her and bring her into interrogation. Patience pleads her case and tries to explain, but as Tom pointed out all the evidence pointed to Catwoman as the killer. Patience realizes that Tom probably still saw her as the girl from his first impression - a crazy woman with suicidal or even hallucinatory tendencies. In tears, she allows him to lock her in the detention cell, but it only takes the appearance of Midnight for her to successfully escape. She ends up going to BeauLine's production warehouse and disabling all the trucks so that the merchandise could not be delievered.
Tom goes to the BeauLine press conference where Laurel is heavily promoting the producting and giving out free samples. He privately takes her aside and indicates that he had evidence about her crimes, and when she unintentionally confesses he is unprepared for her murderous reaction. Luckily Catwoman arrives in time, but Tom is already injured and unable to run quickly out the building. Instead they are forced to stay behind to fight Laurel and her lackeys. Despite his injured state Tom is able to take care of himself, and Catwoman fights her way to Laurel for a final confrontation. To her dismay, it turns out that Laurel is unable to feel pain, largely due to the continuous use of BeauLine. Nevertheless, Catwoman manages to damage skin and eventually causes Laurel to plunge out the window. For the first time, Catwoman realizes she may have killed someone.
Catwoman's exploits end up on the news, and the Hedra company is shut down, but partially because of the ambigous nature of Catwoman and partially out of guilt Patience is unable to relax. She breaks up with Tom and prepares to live her own life, free and untamed, the Catwoman of her generation.