Gary (Anthony Michael Hall) and his nerdy friend Wyatt (Ilan Mitchell-Smith) are two dateless teens stuck in Wyatt's rather posh suburban family home on a Friday night. On a whim, Gary suggests that they use Wyatt's powerful computer to "make a woman" (or at least a somewhat realistic simulation that they can interact with). They begin to feed various images and other data into the computer, but are unsuccessful at first. After hacking into a powerful Air Force computer system, a weird electrical storm develops. The two teens try to unplug the computer and stop the rather supernatural events, but to no avail. Finally, a real woman (Kelly LeBrock) whom Wyatt names "Lisa" emerges from the bathroom, much to their shock and delight.
The hapless duo have limited fantasies about what they would do with a girl, should one ever come near them, but Lisa has plans of her own. She magically provides a car, fake IDs, and cool clothes for them, and they head to a Blues Club. They actually get along quite well with the much older Black and Latino patrons, once they down a few "Blind Dog Bourbon" shots. Once home, they are confronted by Wyatt's older Brother Chet (Bill Paxton), who is a stern and unsympathetic character with a military school bearing. After offering Chet a bribe to leave him alone, Wyatt looks to be on the way to losing his virginity -- but passes out from exhaustion upon hitting his bed.
The next day, Gary and Wyatt are provided with a black Porsche, and head to the mall. Here they are confronted by Ian (Robert Downey, Jr.) and Max (Robert Rusler), two "cool kids" with pretty girlfriends Deb (Susanne Snyder) and Hilly (Judie Aronson) . At first the two nerds are at a loss, but when it becomes clear that they are with Lisa, the more popular kids are left flabbergasted. Lisa announces that she wants to have a party, which nearly causes Wyatt to have a meltdown. Still, there is no stopping her, and she insists on meeting Gary's parents too. Eventually the party starts, and because of Lisa's magic, the least-popular duo in the school attract a crowd of hundreds.
Lisa carefully steers events so that the normally-shy teens are forced to appear tough and in-charge under pressure. The fickle Deb and Hilly end up making out with Gary and Wyatt, and things are looking up. Chet meets and offends the strong-willed Lisa the next morning, and she turns him into the nasty little monster he actually is. As Wyatt's Parents return Sunday afternoon, Lisa magically (ala Cat in the Hat) repairs the party damage, changes Chet back, and then vanishes herself. Despite having created the "perfect" woman, whom they supposedly "control," neither Gary nor Wyatt actually gets more than a kiss from her.