Sue Ellen Crandell (Christina Applegate) is a 17-year-old high school dream teen who looks forward to the summer when her divocred mother will be in Australia all summer. Sue Ellen tells her friends at the clothing shop about her thoughts of a fun-filled summer of freedom in the rural Los Angeles suburbs while the rest of her friends are planning a trip to Europe. After arriving home, Sue Ellen's dreams are then abruptly curtailed upon the arrival of elderly Mrs. Sturak (Eda Reiss Merin), whom Mrs. Crandell (Concetta Tomei) has hired as a live-in babysitter for Sue Ellen and her younger siblings: 16-year-old Kenny (Keith Coogan), a heavy metal-loving stoner; 12-year-old Zach (Christopher Pettiet), a romantic ladies' man; 10-year-old Melissa (Danielle Harris), an athletic tomboy; and eight-year-old game show addict Walter (Robert Hy Gorman).
Mrs. Sturak reveals her true colors the moment Mrs. Crandell departs: an iron-fisted, whistle-blowing, no-nonsense disciplinarian. That evening, Sue Ellen inspires her siblings to rise up against Mrs. Sturak and end her reign of terror. The moment they move in to confront her, however, they discover Mrs. Sturak dead in her chair. The Crandells panic and debate calling the police, or their mother in Australia, but Sue Ellen realizes that doing so jeopardizes any chance for summer fun for their mother will no doubt return and unfairly blame them for Mrs. Sturak's death. They finally decide to seal Mrs. Sturak's body in a trunk, and drop it off anonymously at the local morgue with a note reading: "Nice old lady inside. Died of natural causes."
More panic ensues when the kids realize that all the cash their mother left them for the summer was in an envelope that Mrs. Sturak kept about her person at all times. The Crandell kids now have no money, but Sue Ellen decides all she needs is a job. Unfortunately, the only place that hires her is Clown Dog, a fast-food restaurant, where she performs grunt labor in miserable conditions. Despite meeting Bryan (Josh Charles), a handsome and supportive co-worker who becomes her boyfriend, Sue Ellen quickly gets fed up with Clown Dog and quits.
Sue Ellen then forges the résumé of a mid-level fashion executive and poses as a 28-year-old as she applies at General Apparel West, a downtown L.A. clothing manufacturer, for an open receptionist position. Although scorned by Carolyn (Jayne Brook), the rude and sarcastic receptionist due for a promotion, Sue Ellen's résumé garners the attention of Rose Lindsey (Joanna Cassidy), the senior vice president of operations at GAW, who offers an executive administrative assistant position to her.
Sue Ellen struggles to keep the truth about her double life hidden from Bryan and GAW. But things become more complicated after Sue Ellen learns that Bryan is the younger brother of Carolyn the receptionist who quickly sees through Sue Ellen's charade and she teams up with her equally cynical and sneaky boyfriend Bruce (David Douchney) to try to prove that sue Ellen is a phoney to the skeptic Rose.
After learning about office money called petty cash, Sue Ellen "borrows" enough for groceries. She convinces herself that she will be able to pay back the money she stole, but on her first payday, she sees that her paycheck is only $730, for nearly half of her pay is taken away for taxes and her siblings used even more petty cash, totalling over $3,000, to purchase nonreturnable items such as Zach purchasing a huge stereo TV and sound set.
Compounding her distress is bad news from Rose: the company is going under because GAW's dowdy fashions aren't selling amid their buyers, and soon all of them will be out of a job. In a moment of epiphany, Sue Ellen goes out on a limb for GAW by designing high-end fashions on her own for GAW. She convinces Rose and the other senior staff members that her hip clothes could pull GAW back out of bankruptcy. Rose is impressed and wants to hold a gala event-type show to reveal GAW's new fashion line to their buyers. Realizing there isn't enough petty cash to rent a hall, Sue Ellen convinces Rose to hold the fashion show at the Crandell house.
Working together with her siblings and friends, they use the remaining petty cash to spruce up the house and host the event themselves. The evening goes off swimmingly, with the audience loving the new GAW clothing line. Right at the show's finale, however, Bryan shows up unexpectedly, quickly followed by a furious Mrs. Crandell, back early from Australia (having not heard from the kids over their refusal to take her phone calls).
Overwhelmed by her lies, Sue Ellen announces the truth to everyone and apologizes to Rose, but Rose assures her that the buyers loved the new fashions and don't care about Sue Ellen's personal life. With GAW's future secured, Rose gladly offers the "real" Sue Ellen another position at GAW, but Sue Ellen declines in favor of pursuing college first.
With the party ended and the guests departing, Mrs. Crandell begins to lose her temper, but is quickly calmed down by a now more-mature Sue Ellen, as well as the suttle Kenny, Zack, Melissa and Walter. Mom walks around the house, impressed and supicious by the cleanliness, expensive new furnishings, and the behavioral turn-around of all her children. As Sue Ellen and Bryan embrace and kiss, Mrs. Crandell asks about the whereabouts of Mrs. Sturak.
The credits then begin to roll as the film cuts to a scene at the local cemetery, where the two morgue attendants are visiting Mrs. Sturak's grave and saying how nice it was of her to leave them all her money which they squandered on gambling trips to Las Vegas. Her tombstone simply reads: "Nice Old Lady Inside. Died Of Natural Causes."