32-year-old Sonny Koufax (Adam Sandler) is a law school graduate who is steadfastly avoiding the pressures of adult responsibilities. As the lone holdout among his group of law school buddies, he eschews the career and marriage fast track for a part-time job, the neighborhood sports bar and endless take-out meals.

Even his roommate and old law school buddy, Kevin (Jon Stewart), is joining the ranks of the grown-ups by proposing to his longtime girlfriend, Corinne (Leslie Mann), who’s eternally at odds with Sonny and whose former life as a Hooters waitress is always good for a chuckle. The only people who appreciate Sonny’s unique outlook on life are Corinne’s beautiful sister, Layla (Joey Lauren Adams), a hard-working attorney, and Sonny’s longtime take-out food delivery guy (Rob Schneider), who’s practically achieved the status of a third roommate.

In a misguided attempt to impress his girlfriend, Vanessa (Kristy Swanson), and prove he’s ready for responsibility, Sonny adopts a five-year old boy, Julian (Cole and Dylan Sprouse), under the false pretense of being his biological father. When the plan doesn’t work out and Vanessa leaves him anyway, Sonny tries to “return” the kid to Social Services. Faced with placing Julian in a children’s home until the agency finds a suitable foster family, Sonny agrees to take care of him for the time being.

Although plunged into the unknown territory of sleep deprivation, irritating children’s songs and bedwetting, Sonny soon embraces his role as a temporary dad taking an unconventional approach to child rearing. But the ruse eventually catches up with him when Social Services discovers he isn’t Julian’s real father. In an attempt to keep the boy he has grown to love and make their relationship legal and permanent, Sonny surprises himself, his family and friends as he comes to learn and accept real-life responsibility.