In 1978, childhood friends Lenny Feder, Eric Lamonsoff, Kurt McKenzie, Marcus Higgins, and Rob Hilliard win their junior high basketball championship. They celebrate at a lake house with their coach Robert “Buzzer” Ferdinando.
Thirty years later, Lenny is a successful Hollywood talent agent, married to fashion designer Roxanne and has three children, Greg, Keithie, and Becky. Eric claims to co-own a lawn furniture company and has two children, Donna and Bean. His wife, Sally, still breastfeeds Bean, who is four. Kurt is a stay-at-home father and has two children, Andre and Charlotte. His wife, Deanne, is pregnant with their third child, and her mother, Ronzoni, lives with them. Rob is married to his much older fourth wife, Gloria, and has three estranged daughters, Jasmine, Amber, and Bridget. Marcus is a slacker and lothario.
When Buzzer dies, the five friends reunite for his funeral, returning to their hometown with their families. Lenny rents the lake house for everyone to stay over Fourth of July weekend, though his family is leaving early to attend Roxanne’s fashion show in Milan. He pushes his spoiled sons to play outside and runs into his childhood nemesis Dickie, who claims Lenny’s foot was out of bounds when he made the winning shot. As the friends spread Buzzer’s ashes, Rob breaks down in regret over his failed marriages, and reveals that he has invited his daughters to visit. The men play “arrow roulette”, shooting an arrow straight into the air and Rob wins by not running for cover, but the arrow impales his left foot. Lenny is thrilled to find the kids playing with cup-and-string telephones; Roxanne realizes the positive impact the weekend is having on their children, and tells Lenny to cancel their Milan trip and stay at the lake instead.
Everyone visits Water Wizz water park where Marcus flirts with Jasmine and Amber after buying them skimpy bikinis, and Eric teaches Bean to drink real milk. The families cause chaos throughout the park: Rob assaults slide attendant Norby when he insults Bridget; Eric ignores Donna’s warning about a chemical in the children’s pool that turns urine blue; and the wives attract a bodybuilder, then jeer at his high-pitched Canadian accent. At the Torture Track, Lenny and his friends meet Dickie’s group and Wily who is severely injured after crashing into a shed while sliding down the Torture Track by his feet.
Returning to the lake house, Lenny teaches his son to shoot a bank shot, and the couples end the night dancing together.
The next day, Rob attacks Marcus, mistakenly believing that he slept with Jasmine, and Marcus admits to feeling insecure compared to his happily married friends. Everyone comes clean about the state of their lives: Roxanne confronts Lenny for canceling their flight to Milan before they left home, and he admits that he wanted their family to have a normal vacation; Deanne confronts Kurt for spending time with the Feders’ nanny Rita, but Kurt retaliates by pointing out how she underappreciates him; Eric reveals that he was laid off from his job; and Rob admits what everybody already knows – that he wears a toupee. Gloria helps everyone reconcile, and Lenny and Kurt offer to help Eric start a new business.
On their last day at the lake house, Lenny and his friends agree to a rematch against Dickie, Robideaux, Muzby, Tardio, and Malcolm. The game culminates in Lenny and Greg facing Dickie and his son, but Lenny misses the game-deciding shot.
As the families watch the Fourth of July fireworks, Lenny tells Roxanne that he let Dickie’s family win to get him off his case, and felt that his own family needed to know what losing feels like. A drunken Marcus plays another game of arrow roulette, and the crowd flees in panic. Trapped in a full-body cast, Wily is struck in the foot by the arrow declaring “We win again!” before fainting.