Greg Focker, a dedicated Jewish American nurse from Chicago, has big plans to propose to his girlfriend, daycare teacher Pam Byrnes. He aims to secure her father's blessing during the wedding of her sister, Debbie, taking place at their parents' house on Long Island. However, his romantic intentions hit a snag when the airline loses his luggage, which just so happens to contain the engagement ring he intended to present.
Upon arriving at the Byrnes family home, Greg encounters Pam's father, Jack, her mother Dina, and their cherished cat, Jinx. Although Jack initially presents a friendly front, he quickly reveals a deep-seated suspicion towards Greg, especially regarding his career choice as a nurse. In an attempt to woo Jack, Greg gifts him a rare flower, but Jack fails to recognize it despite his past as a florist.
Tensions escalate when Jack subjects Greg to an unplanned lie detector test. Pam tries to clarify that Jack's façade as a florist is merely a cover for his true past as a CIA operative, which included interrogating double agents. As Greg interacts with Pam's extended family and friends, including Debbie's future in-laws, he grapples with feelings of alienation. This is compounded by the fact that he learns about Pam's former engagement to Kevin, an attractive, affluent, and friendly man who still maintains a good rapport with Pam, serving as the Best Man at Debbie's wedding.
Despite his best efforts to gain their approval, Greg's clumsy mistakes make him a source of amusement for Pam's family. He unintentionally injures Debbie during a pool volleyball game, causes a sewage flood in the backyard, destroys an urn containing Jack's mother's ashes, and even ignites the wedding altar. Jack becomes increasingly suspicious of Greg, particularly after Greg mistakenly discusses the marijuana interpretation of "Puff, the Magic Dragon." When Jack catches Pam's brother Denny with a marijuana pipe, Denny awkwardly claims it belongs to Greg.
Things take a turn for the worse when Greg loses Jinx and temporarily replaces him with a lookalike stray cat, to disastrous results. The charade is soon discovered when a neighbor finds the real Jinx, leading the entire Byrnes family to demand Greg leave Long Island.
In a heated confrontation, Jack accuses Greg of lying about his Medical College Admission Test, claiming the CIA has found no record of someone named Gregory Focker. In his defense, Greg exposes Jack’s covert dealings, noting that he has seen him in clandestine meetings and conversing in Thai. Jack, however, angrily clarifies that he was merely planning a surprise honeymoon in Thailand for Debbie and her fiancé, Bob.
Feeling defeated, Greg heads to the airport to return to Chicago, but finds himself detained by security for refusing to check his oversized suitcase and for taking his frustration out on a stewardess. Back at the Byrnes home, Pam shows her parents Greg's MCAT transcripts, which reveal that the reason the CIA couldn't find him is that his actual name is Gaylord, not Gregory.
While Jack remains adamant that Greg is not a suitable match for Pam, Dina confronts him about his constant disapproval of every man she brings home. After hearing Pam's heartfelt phone call to Greg, in which she apologizes for not defending him, Jack realizes the depth of Pam's feelings.
In a rush to ease the tension, Jack races to the airport to clear things up with Greg. They both confront their fears and expectations; Greg admits to his worries about reaching Jack's lofty standards. After an assurance of the love and commitment he has for Pam, Jack finally warms up to Greg and gives his approval to be his future son-in-law.
Once they return to the Byrnes home, Greg seizes the moment and proposes to Pam, while Jack and Dina eavesdrop from their bedroom, now eager to meet Greg's parents. Following Debbie's wedding, Jack indulges in hidden camera footage of Greg, where he candidly shares his frustrations with Jack and reveals Denny's marijuana use, marking a comical yet revealing conclusion to a whirlwind family gathering.
