The film begins by showing the Beindorfs, mother Janet (Jamie Lee Curtis), father Ned (Kevin Pollak), Gregory whose nicknamed "Grover" (Kyle Howard), and Stacy (Amy Sakasitz), a supposedly happy family living a typical family life in the suburbs of Defiance, Ohio. It is revealed that Janet and Ned are not happy and in fact separating although they tell their children it is not a divorce. Groves's friend Matt tells him that usually it starts with a separation and ends with a divorce. He asks Groves to check for tell tale signs like who is moving out (usually mom moving out is temporary), if they are not giving reasons then there is a lot wrong and if the suitcases are already packed to estimate the seriousness of the situation. Grover is being bullied at school by TJ.It is Janet and Ned's marriage anniversary and Grover and Stacy first try to recreate their parents' honeymoon in the basement of their house, but this fails to bring any happiness into their relationship. The children then leave the basement telling their parents they must get another surprise for them upstairs. They go up, close the door, and nail it shut. They vow to keep it shut until their parents work out their problems. They removed all the tools, boarded up the windows and cut the phone line to the basement as well. The first night, Janet and Ned only spend time trying to escape the basement.The next day, the kids call in their parents sick at work and then go to school, as usual. Chief Rocco (Ray Walston) is the next-door neighbor to the Beindorf and is a retired police chief. Grover tells his best friend Matt Finley (Mooky Arizona) what he has done and T.J. Krupp (Russel Harper), the wealthy local bully, overhears the conversation. Matt goes over to the Beindorfs' house to look at the children's work and is impressed. T.J. shows up to have a look and actually installs a newer, more secure door to keep the parents trapped. TJ is almost an adult in size and is able to push Groves's parents back while Groves and Stacy install the new steel door, after removing the old wooden door. TJ and Matt then leave to collect their parents and bring them to the Beindorfs' house to lock them up as well. Matt's father Vic (Wallace Shawn) never keeps a wife for more than two years and T.J.'s father Donald (Christopher McDonald) does not treat his wife Gwenna (Sheila McCarthy) well. Matt also brings his bulldog Cosmo and his two younger brothers, Teddy and Jimmy (who come armed with sleeping bags) and T.J. brings Spot, his pet boa constrictor. When Grover asks what is going on in response to his friends setting up camp at his house, T.J. replies with "Our parents could be down there for months!" The next morning Grover goes to school, and Stacy serves breakfast to all parents in the basement.Janet and Ned almost talk Grover into letting them all out but T.J.'s father, Donald (who's an attorney), threatens him with legal action. Grover finds out that his dream girl, Brooke Figler (Jennifer Love Hewitt), is also having parental problems: her mother Cindy (Jennifer Tilly) acts like a teenager, going so far as to trying to hang out with Brooke's friends. Grover invites her to lock up Cindy with the rest of the parents. The children lock up all of their parents and begin to help them solve their problems. The kids lower a TV camera and speakers by cutting hole in the basement ceiling and having a group therapy session for the parents. The parents try to find a way out of the basement (when Ned finds the blueprints to an old laundry chute) while getting along and seeing what each of their problems are.The children also work out their differences with each other above in the house. They work together to clean up the house and act responsibly while their parents are locked up. As they get to talking, they also defend their respective parents and their quirky habits. Rocco continues to keep an eye on the house. Ned and Janet have a fight in the basement as Ned wants to cut off electricity to the house (fuse box is in the basement), while Janet feels that is negative reinforcement. The rapid turning on and off of lights is witnessed by Rocco from across the street, who calls the cops. The cops only find kids having a pajama party with TJ acting as a grown up, and leave.Janet tries to climb up the laundry chute but cannot hold on and slides right back down.The next morning, Janet's lawyer brings over the divorce papers and Grover realizes that his parents had been lying to him. Even Ned did not know that Janet had asked her lawyer to draw up divorce papers. Ned and Donald dig through the cemented window and push Vic through the hole into the backyard where Rocco sees him and calls the police again. Grover is dejected and realizes that what he did was wrong. The kids eventually give in and give up to the police and their parents are set free.It is revealed at the end that Grover's parents reconciled and took a second honeymoon to Hawaii. Matt's parents' wedding lasted past the two-year mark, and they are expecting another child. T.J.'s parents got divorced though his mom later went back to law school and they opened up a law firm together. Brooke's mother started dating other men instead of intruding on Brooke's dates. Also, Grover and Brooke became boyfriend and girlfriend, and she passionately kisses him in front of their classmates at school for a very long time, causing a slightly awkward moment for the bystanders. However, he concluded if their parents ever try to get divorced again, he might think of locking them in the attic.