Sue Ellen "Swell" Crandell, a spirited 17-year-old high school graduate from Los Angeles, faces the disappointment of not being able to join her friends on a summer trip to Europe. With her divorced mother embarking on a vacation to Australia with her boyfriend, Swell eagerly anticipates a carefree summer alongside her siblings: Kenny, Zach, Melissa, and Walter. However, their excitement quickly dims when their mother employs a live-in babysitter, Mrs. Sturak, an ostensibly kind elderly woman who reassures Mrs. Crandell that she can deftly manage all five children.
Once Mrs. Crandell departs, the true, tyrannical nature of Mrs. Sturak is revealed, leading to immediate conflict with the kids. Tragically, Mrs. Sturak suffers a heart attack after stumbling upon the shocking, risqué posters and garish decorations in Kenny’s room. In a panic, Swell discovers the lifeless body, igniting fears that their summer fun will come to an abrupt end if they alert the authorities. Thus, the clever kids devise a plan to conceal Mrs. Sturak's body in a trunk, delivery it to a local mortuary with a comical note stating, "Nice Old Lady Inside, Died of Natural Causes." To their horror, they soon find out that the cash their mother had left for the babysitter went along with the body.
In need of money, Swell takes a job at the fast-food joint Clown Dog, where she catches the attention of her colleague, Bryan. Unfortunately, she quits due to the obnoxiousness of their manager. Instead, she crafts an elaborate fake résumé claiming to be a graduate of Vassar and lands a position at General Apparel West (GAW) as a receptionist, impressing company executive Rose Lindsey on her first day, much to the disappointment of Carolyn, the previous candidate.
Navigating her newfound responsibilities, Swell juggles the adult obligations of her job while trying to relish her teenage years. To help out at home, she borrows her mother's Volvo for work, dipping into petty cash with intentions of repaying it once her paycheck arrives. Her budding romance with Bryan faces challenges as she discovers he is Carolyn's brother. Tensions escalate, leading to a heated argument that drives a wedge between them.
Meanwhile, Carolyn, fueled by jealousy, plots with her colleague Bruce to undermine Swell at work. Swell must also fend off unwanted advances from Gus, a charismatic yet disloyal executive in a relationship with Rose. The situation worsens when Swell learns that her siblings have already taken some of her petty cash to buy extravagant gifts, sinking her further into a moral quandary.
When GAW faces potential bankruptcy, Swell seizes the initiative to launch a new clothing line, with Rose supporting the idea of a fashion show to showcase the designs. Lacking funds for a venue, Swell persuades her siblings to help tidy up their home and act as caterers for the event. During the gathering, tensions flare as Rose ejects Gus after Swell exposes his indiscretions. Just when Carolyn tries to sabotage Swell by revealing her age with a copy of her driver's license, Rose dismisses these antics as jealousy.
The event appears to flourish, gaining the interest of fashion buyers, until an unexpected twist disrupts everything. Bryan, hoping to mend fences with Swell, arrives in a truck only to find Carolyn there. At this crucial moment, Mrs. Crandell returns home earlier than expected, forcing Swell to confess her entire deception. Thankfully, Rose empathizes with Swell's situation, recognizing the potential success stemming from the fashion show. In an ironic turn, Carolyn and Bruce discover their vehicle has been vandalized by Kenny’s friends.
As Rose extends an offer to Swell for a personal assistant role, she politely declines while contemplating college instead. After a heartfelt apology to Bryan, the two reconcile with a kiss, interrupted by Mrs. Crandell's inquiry about the late Mrs. Sturak.
In a humorous closing scene during the credits, two mortuary workers survey Mrs. Sturak’s gravestone at the cemetery, contemplating a trip to Las Vegas with the cash discovered with her body. The gravestone comically reads: "Nice Old Lady Inside, Died of Natural Causes."
