In the sweltering heat of her 17th summer, Sue Ellen "Swell" Crandell finds herself in a predicament, trapped at home while her friends embark on exciting European adventures. Just as she hoped for a liberating summer with her siblings Kenny, Zach, Melissa, and Walter, their plans are dashed as their mother leaves for an exhilarating trip to Australia with her new partner. In her absence, the children are left under the watchful eye of the seemingly kind-hearted Mrs. Sturak, who quickly reveals herself to be a tyrant, igniting rebellion among the siblings.
Their lives take a dark turn when Mrs. Sturak unexpectedly passes away, forcing Swell to confront a grim scenario. Fearing their mother's return will put an end to their newfound freedom, the siblings hatch a reckless plan to dispose of the body, intending to leave it at the local mortuary along with a fabricated note to cover their tracks. However, their plan takes an unexpected twist when they find that Mrs. Sturak had left behind her purse, stuffed with their mother's spending money. With financial struggles looming over them, Swell decides to take on a part-time job at Clown Dog, where she soon catches the eye of her coworker Bryan.
But after a falling out with an intolerable manager, Swell devises an audacious scheme involving a fake résumé, masquerading as a Vassar-educated young fashion designer in her application to General Apparel West (GAW). Intriguingly, her deception pays off when Rose Lindsey, a powerful executive at GAW, is easily charmed by Swell's fabricated credentials and hires her as an executive assistant on the spot. This unexpected promotion stirs discontent, particularly from Carolyn, who was initially supposed to take on the position, igniting a fierce rivalry.
As Swell navigates her new responsibilities, trying to balance the weight of adulthood with her teen spirit, she resorts to borrowing from petty cash, intending to replace the funds once her paycheck arrives. Meanwhile, her romance with Bryan begins to flourish until she discovers that he’s Carolyn's brother, leading to a heated argument that shatters their budding relationship. Swell faces the fallout of her actions, grappling with uncertainty about her future amidst brewing tensions.
Carolyn, harboring jealousy over Swell’s rapid ascent, colludes with Bruce, another co-worker, to undermine her authority. Meanwhile, Swell also has to navigate the unwelcome advances of Gus, a self-absorbed corporate executive who frequents Rose's company. To add to her stress, Swell discovers her siblings have taken money from her purse to support their lavish spending, which includes purchasing a high-end entertainment system.
When the survival of GAW hangs in the balance, Swell resolves to create a new clothing line, inspired by Rose’s idea of hosting a fashion show. However, the absence of petty cash threatens their plans. In a stroke of brilliance, she suggests converting her home into a banquet hall, with her siblings grudgingly agreeing to help clean and prepare for the event.
The night takes a dramatic turn when Swell’s actions force Rose to confront Gus about his disloyalty, leading to a raw phone call that results in a breakup. To sabotage Swell, Carolyn attempts to discredit her by presenting Rose with a copy of Swell’s driver’s license, revealing her youth. Yet, Rose dismisses this as mere jealousy and chooses to recognize and celebrate Swell's innovative spirit instead.
As the night concludes, Bryan reappears in Swell’s life, looking to mend the rift between them, but his unexpected encounter with Carolyn complicates things further. Just as tensions rise with Mrs. Crandell's imminent return, Swell decides to come clean about her deception to the gathered guests. Despite feeling betrayed, Rose chooses to forgive Swell, acknowledging the evening's success and the enthusiasm from fashion buyers as reasons for her mercy.
In a surprising twist, Carolyn and Bruce's vehicle is vandalized, leaving them in shock. Swell declines Carolyn's offer to become her personal assistant, opting to focus on her studies instead. Bryan and Swell share a poignant moment of reconciliation, interrupted by Mrs. Crandell's sudden questioning about what happened to Mrs. Sturak.
As the credits start rolling, the film reflects on the life of Mrs. Sturak, her gravestone humorously engraved with the words: "Nice Old Lady Inside, Died of Natural Causes." Two mortuary workers ponder using the money found on her person for an impromptu trip to Las Vegas, leaving the audience with a blend of laughter and nostalgia.
