The film kicks off with Pete (Paul Rudd) and Debbie (Leslie Mann) engaging in a rather intimate moment in the shower. However, when Pete admits to taking a Viagra pill given to him by his friend Barry (played by Rob Smigel), Debbie's anger flares and their romantic moment quickly dissipates. The underlying tension is further heightened as Debbie grapples with the reality of turning 40.
Fast forward five years since the events of Knocked Up, Debbie is now the proud owner of a boutique store managed by Desi (Megan Fox) and Jodi (Charlyne Yi), while Pete runs his own record label, struggling to stay afloat alongside friends Ronnie (Chris O'Dowd) and Cat (Lena Dunham). Their relationship faces additional challenges as they navigate parenting their two daughters, Sadie (now 13) and Charlotte (8 years old). In an effort to celebrate Debbie's birthday, the couple embarks on a romantic getaway, where they find themselves indulging in marijuana cookies and whimsically hypothesizing about what it would take to eliminate one another.
After a candid discussion with her friends Jason (Jason Segel) and Barb (Annie Mumolo), Debbie resolves to enhance her marriage and strengthen family bonds through exercise and communication with her parents. She confronts Pete about the financial strain caused by his repeated monetary assistance to his deadbeat father, Larry (Albert Brooks). Despite her concerns, Pete visits Larry but ends up caving and giving him more money.
In a surprising twist, during a visit to her gynecologist ([Tim Bagley]), Debbie discovers that she is pregnant but chooses to keep this news from Pete. Tensions rise further when Debbie confronts a school bully, Joseph (Ryan Lee), leading to an altercation with his mother, Catherine (Melissa McCarthy). In a meeting with the principal, they both feign ignorance about the previous incidents, which leads to a surprisingly mutual understanding.
One night, while out dancing with Desi, Debbie confronts her suspicions of embezzlement, only to discover that Desi’s lavish lifestyle is the result of her being an escort. Meanwhile, conflicts escalate at home as Sadie and Charlotte's constant bickering spills over into arguments between Pete and Debbie.
On Pete's 40th birthday, a heated argument ensues between him and Larry over finances, while Debbie faces off with her father, Oliver (John Lithgow), regarding his absence in her life. Their confrontation reveals that he has always cared for her, despite differences in their perceptions of life’s perfection. Shortly after, Pete learns of Debbie's pregnancy and, in a fit of rage, storms out on his bicycle, which leads to an unfortunate accident. In the aftermath, both Debbie and Larry gain valuable insights from the ordeal, as Larry reassures her of her importance in holding the family together.
Eventually, Pete approaches Debbie to share his genuine excitement about their third child. Reconciliation follows, paving the way for a fresh start. The story concludes with the couple attending a small concert where Debbie encourages Pete to consider signing Ryan Adams and take the initiative to speak with him about a potential deal.
As the credits roll, viewers are treated to a humorous extended scene where Catherine skillfully improvises witty insults during a meeting with Debbie, Pete, and the principal, leaving a lasting impression of laughter and spontaneity.
