On a certain late evening, Paul Matthews (Tate Donovan), a heartbroken biochemist, finds himself visiting Madame Ruth (Anne Bancroft), a gypsy who reads palms. To his surprise, she tells him that she sees no romantic prospects in his future. Instead, she offers him a vial of "Love potion number 8" and instructs him to dilute it by one thousand percent, claiming that consuming it will render his voice irresistible to women. Skeptical as a scientist, Paul pockets the potion nonetheless. Later, at a bar with friends, he is challenged to flirt with a beautiful woman, but faces rejection in a rather rude manner.
Meanwhile, Diane Farrow (Sandra Bullock), a psycho-biologist working alongside Paul, struggles home in her faulty Alta Pazzoli car that stalls without warning, only to be ticketed by a police officer, Officer (Blake Clarke). Upon arriving home, Diane's occasional boyfriend, Gary Logan (Dale Midkiff), makes a quick visit, leading to a hasty exit after an intimate encounter.
Back at Paul's apartment, his friends send a hooker named Marisa (Mary Mara) to lift his spirits after the bar incident. As they share a drink, Marisa rummages through his belongings, curious about his high-end gadgets, while he carelessly discards the potion sample. Unbeknownst to him, his curious cat finds the trash, licks the potion, and attracts a horde of male felines into the apartment.
The following day, Paul excitedly informs Diane about the potion's unexpectedly impactful test on his cat. They decide to experiment further, using the potion on a female chimpanzee at their lab. To their astonishment, the female's sound triggers a male chimpanzee to smash through a wall, indicating the potion's effectiveness. Through their analysis, they discover that when ingested, it compels the opposite sex to fall for the user while igniting resentment from the same sex. With effects lasting only four hours, they split the remaining potion, agreeing to not communicate for three weeks.
However, on her way home, Diane again meets the same police officer, and using the potion, manages to charm him into asking her out rather than ticketing her. Soon afterward, her insurance agent reinstates her policy after being enchanted by the potion. While out running errands, Diane encounters Enrico Pazzoli (Adrian Paul), who is taken with her apology and insists she’s the most beautiful woman he’s ever seen, giving her a lavish diamond necklace as a gesture. Reluctantly, Diane accepts his invitation to a fancy gala in honor of Prince Geoffrey (Dylan Baker), who quickly falls prey to Diane's potion-aided allure.
At the same time, Paul's experiments yield astonishing results; he realizes that merely speaking can attract women passionately. He capitalizes on this newfound power, even enjoying a night with the members of the Sigma Delta Pi sorority. His reckless adventure lands him in hot water, but Diane comes to his aid after he gets arrested. They both find themselves indulging in their potion-enhanced lives while navigating budding feelings for each other.
Paul attempts to propose to Diane during a planned romantic weekend, but to his dismay, she confesses her love for Gary. Determined to unveil Gary's true nature, he pretends to be merely a friend. As Paul digests this heart-wrenching news, Marisa, the hooker, breaks into his apartment, unwittingly sampling the love potion herself. The fallout is immediate, and they soon realize that Gary has gotten hold of the remainder of Love potion number 8.
In search of something stronger, Paul visits Madame Ruth again, and she presents him with "Love potion number 9" but warns him of the consequences if Diane's affections are not genuine. As the tension builds, Diane’s wedding to Gary looms closer, while Sally (Hillary B. Smith), Diane's friend, reluctantly assists Paul in his quest to gain Diane’s love. The chaotic climax unfolds at the chapel where manipulation ensues, and Paul’s journey comes full circle.
Just as the wedding ceremony begins, chaos erupts when Marisa's potion-enhanced allure bewilders everyone, sending chaos into the crowd. In a fervent attempt to win Diane back, Paul fights through the confusion to kiss her, striving to prove that they are meant to be together. But as further dilemmas arise, it takes a few extra moments for Diane to realize that Paul is indeed her true love—culminating in a heartwarming, albeit chaotic, resolution.
