Tate Donovan, a geek biochemist with no luck at all with women, is persuaded by his friends to visit a gypsy, Madame Ruth. She gives him "Love Potion No. 8", an elixir which can potentially force anyone of the opposite sex to instantly fall in love with him. He shares the potion with his friend, Diane, and together they embark on a "scientific evaluation" that gets out of hand.