Three years after the events of Grown Ups, Lenny Feder has relocated his family back to the small town where he and his friends grew up. This time around, the grown ups are the ones learning lessons from their kids on a day notoriously full of surprises, the last day of school. In the beginning, all of them still think that their children are still kids. But throughout the movie they begin to accept that their children are growing up. In the Beginning, Lenny wakes up to a moose in his home, and chases it out with his family, which in the process, destroys his daughter, Becky's stuffed monkey, Mr. Gigglesworth. After a long lecture from his wife regarding his daughter's ballet recital, Lenny decides to enjoy the town.
He 'borrows' a school bus from his sons' school bus driver, Nick. He catches up with Kurt (Chris Rock), who works for a cable company. He is upset at the time because his wife forgets their 20th Anniversary. They stop by Eric Lamonsoff later to pick him up in the school bus. Lenny releases a slur of insults on a bully in his son, Keithie's school. After dropping all of them off at school, Lenny, Kurt, and Eric go to K-Mart, where Nick strips down to his underpants and socks, and crashes in one of the beds. When asked to leave the bed, he remarks, "Fine", and goes to use a K-Mart toilet right across the room. Meanwhile, Marcus, (Higgy), picks up his son from the train station. One thing led to another, and the 4 wind up at an old place they used to dive off rocks, also where their sons meet an intoxicated sexy college girl, Savannah.
The 4 guys meet some college guys, who make them jump off the rock....naked. Lenny decides to throw a party that night, and he buys juice boxes for the occasion. At the party, Lenny meets his wife on the balcony and asks her about why she was asking him to have another kid, since she was asking him throughout the day. She starts to cry, and Lenny asks if she is having hormone situations. She admits she is going through menopause, as well as she admits she is pregnant, and walks into the room. Lenny, left discombobulated on the balcony, says to himself, "When did we have..?" And after a few seconds of thinking screams into the air, "D*** you Motel 6 Hot Tub!" The party continues, and the college kids show up to crash it. They fail and succeed, and at the end, Lenny and the college kids' leader, who is played by Taylor Lautner, engage in a fight.
Punches fly, mostly at Lenny, who falls to the ground. Lenny's daughter arrives at the scene, and says to the seemingly victorious leader, "You won. Here's a present for you", and hands him Mr. Gigglesworth, who attracts the deer seen in the 1st scene to tackle him. Later in the night, Lenny talks to the baby in his wife's stomach. The film ends with Lenny and Roxanne laying down in bed. As Lenny take off his shirt, and literally jumps into the bed, the screen zooms out of their house by a window, and the Credits roll.