Four years after the original film's events, Annie announces to her family that she is expecting a baby. The patriarch George, who is voiced by the talented Steve Martin, begins to recount the challenges and unexpected experiences of the nine months leading up to this news. As anxiety grips him, he is fraught with the belief that he is far too young to embrace the role of a grandfather. In his quest to cope, he instructs his assistant to compile a list of anyone older than him, orders a dye job for his hair to cover the gray, and following a leaky roof situation, he contemplates selling their family home if any more mishaps occur.
Just two weeks later, as if to toy with his nerves, a termite infestation manifests in their beloved house. In a moment of rash decision-making, George sells the property to the Habibs, refusing to inform Nina, his wife, portrayed by the skilled Diane Keaton. During a dinner with deep family discussions about the baby's potential last name—whether it should bear a hyphen or not—the atmosphere changes abruptly when George discloses the news of the house sale. Nina is incensed, especially since they must vacate in merely ten days without a new place to call home. In a pinch, they must take up residence in the mansion owned by Bryan's parents, John and Joanna MacKenzie, who are enjoying a cruise in the Caribbean. This new living arrangement brings them face to face with the MacKenzies' notoriously aggressive Dobermans, an encounter that unsettles George, who remains haunted by a previous run-in with the dogs.
As the days unfold, Nina begins to experience unsettling symptoms that initially lead them to suspect menopause. However, a visit to the doctor reveals another surprise: she is pregnant too. This twist of fate leads to an unexpected reunion with Franck, their former wedding planner, who bubbles with excitement at the prospect of both women being expectant mothers. On their ride home, George and Nina hold differing views on the reality of becoming parents again. George's perspective shifts once more, now feeling too old to take on this new parental journey. The friction escalates when they visit Annie and Bryan’s home to share the news, leading Nina to confront George about his insensitivity, ultimately asking him not to return home.
An evening walk proves pivotal as George is jolted by the sight of demolition equipment at their old residence, which hints that Mr. Habib has plans to raze the home. Frantic with a sense of loss, George rushes to confront Mr. Habib, pleading for him to reconsider, citing the sentimental value of the house. His drive to reclaim the home grows stronger when he acknowledges his desire to raise his impending child there, which compels him to offer to buy it back. Following a negotiation, Habib agrees on the condition of an upfront payment of $100,000, and a reluctant George submits to the demand. The couple then returns to their cherished house.
A twist arises when Annie shows up at the Banks' house after a quarrel with Bryan, presenting an exciting job opportunity in Boston that ignites further discussion. Despite his reservations about moving so far away with a baby on the way, George, with conflicting emotions, encourages Annie to seize the opportunity. Bryan’s heartfelt apology leads to the decision that they will indeed relocate to Boston.
In the midst of these developments, George enlists Franck once again to orchestrate a joint baby shower for the expectant couples. As the due date approaches, Bryan is called away for an urgent meeting in Japan, leaving Annie in the care of George and Nina. With both women navigating through their pregnancies, the responsibilities of caregiver become overwhelming for George, necessitating assistance from Matty and Franck while he manages work obligations. Franck's enthusiasm turns a simple nursery decoration into an elaborate renovation, which he dubs "the baby's suite." As George faces sleepless nights and mounting pressure, his exhaustion bottlenecks into a need for aid. Franck supplies him with "Vatsnik," a sleeping pill from his homeland, but George unwittingly overdoes it and passes out at dinner, alarming the family further.
When Annie unexpectedly goes into labor, Franck takes the reins to transport the family to the hospital, though a still-groggy George tags along, mistakenly identified as a patient in need of a prostate exam, which snaps him back to reality. As Nina's labor commences shortly thereafter, George’s skepticism about the replacement obstetrician dissipates when she assures him of her extensive experience. In a timely return, Bryan witnesses the birth of their son named George, while Nina brings a daughter, whom they name Megan, into the world. In a heartwarming conclusion, George cradles both newborns, reflecting on his narrative before the night winds down to a new beginning as Bryan, Annie, and little George prepare to embark on their next chapter in Boston.
