One evening, a man presenting himself as Carol's (Shelley Long) long-lost first husband, Roy Martin (Tim Matheson), unexpectedly arrives at the Brady household. The Bradys, overwhelmed by his tale of amnesia and claims of undergoing plastic surgery due to an injury, readily accept him into their lives. Meanwhile, Mike (Gary Cole) has plans to secretly organize a second wedding and renewal of vows for himself and Carol as a surprise for their anniversary. However, Roy's sudden appearance complicates matters significantly.
Throughout his stay, Roy's demeanor is openly antagonistic, showering them with sarcasm that, amusingly, goes unnoticed by the family. The Brady family dynamic is intricate—Mike has three sons from a previous marriage, and Carol has three daughters. Alice, the ever-dedicated housemaid, adds to the household chaos. Among the children, Marcia stands out as the eldest and most sought after by boys, while Jan struggles with her self-image and has no luck in the dating department.
Roy, who seems to have ulterior motives, is overheard speaking on the phone, possibly about a horse-shaped artifact sent to be cleaned by Alice after a grape juice mishap. As his stay progresses, Peter (Paul Sutera), in an effort to find his path in life, starts idolizing Roy, modeling his behavior after him, which leads to frequent predicaments at the architecture firm where Mike works. In an unfortunate turn of events, Peter's admiration causes him to act out during an internship, becoming preoccupied with Roy's antics instead of focusing on his responsibilities. Notably, Mike uncovers that Roy's mention of "the big house" is a euphemism for prison, raising red flags about Roy's character.
In a misguided attempt to impress Marcia, Roy advises Jan to concoct a lie about having a boyfriend. However, Marcia sees through the deception. The kids soon discover that the horse artifact is at a nearby antique store for restoration, a situation that Roy tries to exploit. Just as plans unfold, Alice is disappointed after Roy dismisses her meatloaf, prompting her to add some magic mushrooms she found in his room to his dinner, resulting in a comical trip for Roy.
As the drama unfolds, Greg (Christopher Daniel Barnes) and Marcia (Christine Taylor) find themselves bundled into an attic-sharing arrangement when they both desire more space. Roy’s presence leads them to question their familial ties, which awkwardly stirs up romantic feelings between them that they both strive to keep under wraps.
Bobby (Jesse Lee Soffer) and Cindy (Olivia Hack) embark on a whimsical "Detective Agency" to locate Cindy's missing doll. Their antics inadvertently reveal a photograph that unmasks Roy as Trevor Thomas, a con artist out to snatch a valuable horse statue worth $20 million. With their cover blown, the children relay their findings to Carol, who confronts Trevor. In a climactic twist, Trevor kidnaps Carol, whisking her away to Hawaii in a bid to sell the artifact to Dr. Whitehead (John Hillerman), while also keeping the kids and Alice bound to prevent interference.
Mike, now fully aware of Trevor’s duplicity, collaborates with the police and rushes to rescue Carol and the kids. In Hawaii, Carol escapes and arrives at Dr. Whitehead's estate just before Trevor can execute his plan. The backstory reveals that Trevor was behind a tragic boating accident that claimed the life of Dr. Whitehead's son and Carol’s first husband, adding emotional depth to their confrontation. In a pivotal moment, Mike arrives just in time to thwart Trevor's desperate attempt for payment, resulting in Trevor's arrest.
Dr. Whitehead offers the Bradys a hefty reward for the return of the artifact, but Mike, valuing the symbol of their family's togetherness more, respectfully declines the money. In a touching gesture, Cindy gives her outgrown doll to Dr. Whitehead, showcasing the bonds of family love. Jan, who previously fabricated a fake boyfriend named George Glass, ends up meeting a real boy by the same name during their escapade in Hawaii, kindling a genuine romance.
As the family reunites at home for a vow renewal ceremony, in a generous act, Marcia concedes the attic space to Greg, emphasizing their sibling bond. The film concludes with Carol tossing her bouquet, which a whimsical genie named Jeannie catches, introducing an unexpected twist by claiming to be Mike's first wife, leaving everyone in shock.
