Mel Coplin, portrayed by Ben Stiller, and his wife Nancy, played by Patricia Arquette, are a New York couple striving to adapt to life with their newborn son. They live close to Mel's neurotic adoptive parents, Ed and Pearl Coplin, who are portrayed by George Segal and Mary Tyler Moore, respectively. The couple is faced with a unique dilemma: they refuse to name their son until Mel can uncover the identity of his biological parents. Mel promises Nancy that knowing his roots will lead them to the perfect name, but in the meantime, their romantic life has significantly deteriorated since becoming parents.
When an employee from an adoption agency identifies Mel's biological mother's name in a database, Mel decides it’s time for a personal meeting. The enthusiastic but bumbling Tina, played by Téa Leoni, an adoption agency worker in the midst of her own divorce and yearning for a child, offers to tag along on a trip to San Diego to meet Mel's mother. Tina is simultaneously working on her dissertation about the emotional consequences of reunions, planning to document Mel’s encounter as part of her research.
Left feeling unappreciated, Mel’s adoptive parents voice their disappointment, especially as Ed approaches his upcoming 60th birthday, which he fears Mel might overlook. Pearl struggles to comprehend Mel's yearning to connect with his biological roots, perplexed that he might not view them as "good enough" parents.
However, the much-anticipated trip spirals in unexpected directions, turning into a chaotic journey across the United States. Their first stop introduces them to Valerie, a Scandinavian woman with Confederate ancestry, who is startlingly not Mel’s biological mother. As the flirtation between Mel and Tina grows, Nancy feels threatened by Tina’s past as an exotic dancer, creating a tension-filled atmosphere among them.
Next, they arrive in rural Michigan hopeful to meet Fritz Boudreau, the man listed in the adoption files. Fritz, played by David Patrick Kelly, is a trucker with a violent edge who, upon thinking he might be Mel's father, takes him for a ride in his semi-truck. Chaos ensues when Mel accidentally crashes the truck into a Post Office, leading to a run-in with ATF agents, Tony and Paul (played by Josh Brolin and Richard Jenkins), who just so happen to share a romantic relationship.
The plot thickens as it’s revealed that Mel's biological parents, Richard and Mary Schlichting, portrayed by Alan Alda and Lily Tomlin, had to give him up due to legal troubles stemming from their involvement in the drug trade. The group encounters more hurdles, including a comical yet chaotic dinner that culminates in one of the agents accidentally getting dosed with LSD by Mel's less-than-cordial biological brother, Lonnie.
After a series of misadventures, including attempts to escape by car and the ensuing confusion involving both sets of parents, Mel ultimately finds common ground with both families. Through it all, he decides to name his son Garcia, proudly affirming his bond with Nancy, whom he has come to appreciate even more, along with his adoptive parents. As the story concludes, a montage showcases the evolving relationships within each family, highlighting that despite their challenges, Mel and Nancy are genuinely happy together, forging ahead in their new life as parents.
