On one late evening, Paul Matthews (Tate Donovan), a lovelorn biochemist, visits Madame Ruth (Anne Bancroft), a gypsy who reads his palm and claims she sees no women in his life. She offers a sample of "Love potion number 8," instructs him to dilute it by one thousand percent, and asserts that when he swallows it, women will find his voice irresistible. As a scientist, Paul is doubtful, but he pockets the sample. He goes to a bar with his friends, and they dare him to flirt with an attractive woman, who rudely turns him down.Meanwhile, Diane Farrow (Sandra Bullock), a psycho-biologist who is Paul's co-worker, drives home in her defective Alta Pazzoli car, which cannot stop without stalling, and a policeman (Blake Clarke) tickets her for rolling through a stop sign. When she arrives home, her occasional boyfriend, Gary Logan (Dale Midkiff), stops by to have sex, and quickly leaves.Elsewhere, when Paul arrives home, his friends send a hooker named Marisa (Mary Mara) to make him feel better about being rebuffed earlier at the bar. As he mixes her a drink, Marisa eyes his electronic equipment, and then searches his bathroom's medicine cabinet, sampling various items. Later, she returns to her pimp's car, and announces that Paul merely wanted to talk, and his apartment is full of expensive items. Inside, Paul tosses the love potion sample into the trash. Later, his cat routs through the garbage and licks the potion. She meows and the neighborhood male cats swarm into Paul's house.The next day, Paul tells Diane about the potion's effect on his cat. They try the potion on a female chimpanzee in the laboratory, and when she makes a sound, a male chimpanzee breaks through a wall to reach her. Paul and Diane test the potion under their microscope and discover that when someone swallows it, any sound from their vocal chords will make the opposite sex fall in love with them while angering members of the same sex. The potion must be diluted and the effects last only four hours. Deciding human testing is necessary, Paul and Diane divide the remaining potion and agree not to speak to each other for three weeks.On her way home, the same police officer stops Diane. However, she uses the potion and he asks for a date instead of ticketing her. She receives a notice that her automobile insurance is canceled because of her previous tickets.At the insurance company, Diane sprays the potion on a male agent... who immediately reinstates her policy. Outside, Diane bumps into Enrico Pazzoli (Adrian Paul), captivating him as she apologizes and walks away, pausing to look at a diamond necklace in a store window. Moments later, he catches up with Diane at her car. Insisting she is the most beautiful woman in the world, he hands her the diamond necklace. She refuses to accept it, but changes her mind when he introduces himself and acknowledges his family's company made her car. He invites her to a black tie gala, and purchases a gown for her to wear with the necklace. The ball is in honor of Prince Geoffrey (Dylan Baker), the playboy Prince of England, and he is among the men caught in the spell cast by Diane's potion-enhanced voice. Later, Gary Logan visits Diane's home and is refused entrance by Prince Geoffrey's security officers.Meanwhile, Paul's "research" proves the potion is 100% effective on the opposite sex. He merely has to speak to attract women, and caps his experiment with an evening making love to the members of the Sigma Delta Pi sorority. Although he is arrested for being in a sorority house after hours, he laughs through his mug shot. Diane's potion-enhanced voice gets Paul released.As Paul and Diane leave the police station and get into Prince Geoffrey's limousine, she does not want her voice to affect Paul, so she remains silent while indicating Prince Geoffrey should invite Paul to join them at an event that evening. When they arrive at the party, Diane uses the spray and offers a squirt to Paul. She watches as he moves across the room, effortlessly attracting women. He notices her leave with Prince Geoffrey, and rushes outside as they drive away.The next day, Diane reveals that Prince Geoffrey proposed, but she needs to think about it. She states that she always felt ugly and had not had a date in four years, and now the world's most desirable man finds her attractive. Paul is upset that she forgot they went out to lunch once, which he considered a date. She apologizes for not realizing it was a date. He admits he did not ask for a second date because she mentioned Gary was "kind of" her boyfriend. Diane insists Gary is not her boyfriend, and agrees to date Paul. They go on several dates and fall in love.Paul purchases an engagement ring and plans a weekend getaway with Diane to propose to her. However, when he knocks on her door, there is no answer. When she finally telephones him, she reveals that she is in love with Gary. Upset, Paul decides to pretend to be her friend and make her realize Gary's faults. She meets him, and although she claims Gary is perfect, she admits she does not like everything about him. At that moment, Gary telephones and she agrees to his request to stop seeing Paul.As Paul returns home, Marisa, the hooker, is breaking into his apartment, but pretends she was waiting for him. When he invites her inside, she riffles through his medicine cabinet and samples the potion. It has an immediate effect on Paul, who gives her the remaining potion, as well as his electronic equipment. Four hours later, the effects fade and Paul realizes the potion's strength. Suspecting Gary is using it to control Diane, he contacts Madame Ruth and learns Gary obtained the rest of love potion number 8. He asks for something more powerful and she pulls out "Love potion number 9", but warns Paul to be certain Diane truly loved him. If they both once loved each other, they will love each other again forever. However, if she did not love him, he will love her for the rest of his life and she will hate him. Madame Ruth instructs they must both drink from the same cup and kiss. Five minutes later, he will hear a song in the breeze if it works. If it does not work, he will taste mule sweat.Paul asks his friends for assistance, but they do not believe him about the potion. However, Marisa arrives and announces she is running low on potion. Her voice enthralls Paul and his friends, who hand over their wallets and reveal Gary has the remaining potion. Four hours later, his friends recover from the spell and agree to help Paul. They go to Diane's home, where her friend, Sally (Hilary B. Smith), is picking up items for Diane's wedding taking place in one hour. Sally is to be matron of honor, but is unhappy at Diane's sudden decision to marry Gary. She agrees to help Paul, and they race to the church where Sally will get Diane to drink the potion, and then bring the glass to Paul. Diane drinks from the champagne glass, and Sally reaches for it, but Gary arrives, drinks from the glass and kisses Diane.Paul telephones Madame Ruth for advice, and she tells him to drink from the glass, too, and kiss Diane, who will choose the man she truly loves.Paul sees Marisa arrive to find Gary. He covers his ears as he runs past her, bursts into Diane's room, and drinks the champagne. He tries to kiss Diane, but Gary stops him. As Gary and police officers lead him outside, Paul sees Marisa and covers his ears. However, Gary is enchanted by her voice. He takes Marisa to his car and gives her the remaining potion, warning it should be diluted. Gary kisses Marisa and is confused to taste mule sweat. As he returns to the chapel, Marisa swallows a drop of the undiluted potion. The wedding has begun when she steps inside, but her potion-enhanced cough stops everything. Women become enraged, and the men rush toward her. As Marisa runs away, every man follows her.At the police station, officers see the crowd of men following Diane, and Paul covers his ears as she runs by. Moments later, Paul is the lone man in the station. Hundreds of men chase Marisa into an alley and corner her. It is here that Marisa discovers when she orders them to stop, and learns she can make them do whatever she wants.Meanwhile, Paul rushes to the chapel and kisses Diane. However, Gary returns and she runs into his arm. Paul punches him and walks outside. He waits five minutes for Diane, but it takes six minutes for her to realize that Paul is the love of her life.