Mac and Kelly Radner are on the brink of welcoming their second child and are getting ready to sell their home to the Baiers family. Their realtor reminds them that the sale is in escrow for 30 days, adding pressure to their situation. Meanwhile, Mac’s friend Jimmy and his wife Paula are also expecting a baby. In a twist of fate, three college freshmen — Shelby, Beth, and Nora — realize that sororities aren’t allowed to host parties and are turned off by the sexist culture dominating fraternity events. This leads them to form their own sorority, Kappa Nu, with the intention of creating a space where they can hold their own parties.
Teddy Sanders, the charismatic host of the current poker night, reunites with his old fraternity brothers. Each has traveled a unique path since graduation: Pete has made a name for himself as a successful architect while embracing his identity, Scoonie is busy managing his own mobile app, and Garf has recently started his career as a rookie cop. However, Teddy's life has taken a downturn; burdened with a criminal record, he struggles to find meaningful employment.
The friends come together to assist Pete's boyfriend, Darren, in proposing, but their successful plans lead Pete to ask Teddy to move out. Upset by this turn of events, Teddy goes back to the old frat house, coincidentally close to the Radners. There, he encounters Shelby, Beth, and Nora, the very girls who want to rent the house for Kappa Nu but lack sufficient funds. Seizing the opportunity to regain his sense of purpose, Teddy offers to help the girls throw parties, which garners enough donations and recruits to cover their rent.
Concerned that the presence of the sorority might scare off the Baiers, Mac and Kelly plead with Shelby to hold off on the partying, but she defiantly refuses. The Radners then approach Dean Gladstone for assistance, but he admits that there’s little he can do regarding an independent sorority. Even Shelby’s father turns out to be unhelpful, prompting the girls’ prank wars against the Radners. In a retaliatory move, Mac, Kelly, Jimmy, and Paula infest the Kappa Nu house with bed bugs, forcing the girls out for mandatory fumigation just as the Baiers arrive.
As tensions peak, Teddy and Pete argue about their respective lives. Unable to afford rent, the girls contemplate selling marijuana at their tailgate party, which leads to chaos as they get their competition arrested. When Teddy objects to this plan, he gets kicked out and seeks refuge with the Radners to help take down the sorority. They infiltrate the party, with Teddy serving as a distraction while Mac pilfers their marijuana. Realizing Teddy is lost and directionless, Mac and Kelly invite him to stay with them.
In a dramatic turn, the girls switch Mac and Kelly's phone numbers with theirs and send a misleading text that tricks Mac into booking a flight to Sydney, Australia. Returning home, Mac and Kelly discover that Kappa Nu has broken into their house, causing robbery and damage, alarming the Baiers who threaten to withdraw their offer unless the sorority is evicted.
Faced with eviction, Shelby reluctantly plans a risqué frat-style party to raise funds. While Mac and Kelly bide their time to call the police, Jimmy and Paula sneak into this chaotic celebration. Amidst mayhem, Teddy grapples with shutting down the power while Shelby unexpectedly locks Mac and Teddy in the garage; however, they escape by using airbags. Hearing the confrontation between Beth, Nora, and Shelby about staying true to their original ideals, Kelly inspires the girls not to lose faith in their mission.
In the midst of this turmoil, Paula goes into labor, and Teddy reconciles with Pete and Darren, agreeing to stand by them as their best man. The girls pivot the party into a more empowering event, and they’re pleasantly surprised to find a wave of interest from girls from other sororities wanting to join Kappa Nu. With new members and a surge of funds, the sorority secures their home, prompting the Radners to reconsider and ultimately rent their house to Kappa Nu, ensuring a better financial outcome than simply selling.
Fast forward three months, and Teddy has transformed into a successful wedding planner specializing in LGBTQ+ ceremonies, preparing Pete for his big day. Mac and Kelly have settled into a new home, enjoying their newfound privacy and reflecting on their parenting journey, especially now with their new daughter, Mildred, who they welcome home to meet Jimmy and Paula’s newborn son, Jimmy Jr.
