Teenagers Matt and Casey are a study in contrasts. The virginal Matt (Chris O'Donnell) has his mind on his studies and his upcoming college career while the beautiful Casey (Drew Barrymore) is an extroverted free-spirit, despite the efforts of her very strict parents. It turns out, however, that opposites do attract, and the couple's intense desire for each other causes the normally level-headed Matt to embark spontaneously on a road trip to Mexico with Casey. But as their journey progresses, Matt begins to realize that the often outrageous, even dangerous behavior of his girlfriend is actually the manifestation of a mental illness--and that he wants her out of his life.