As school ends for the summer, Gerry Garner (Aaron Schwartz) is sent by his parents to Camp Hope, a weight loss camp for boys. Despite worrying at first, Gerry makes friends easily at camp and learns that Camp Hope won't be nearly as bad as he thinks because as veteran camper Roy (Kenan Thompson) put it, "You're not the fat kid, everyone's the fat kid"). He also discovers that the other campers have smuggled in enough junk food to easily stave off the hunger pangs and probably counteract any weight loss that the camp programs cause.
But all is not well at Camp Hope. The first night of the summer brings the revelation that the original owners of Camp Hope, the Bushkins, (Jerry Stiller and Anne Meara) have entered bankruptcy and the camp has been bought by fitness entrepreneur Tony Perkis (Ben Stiller), who announces his plan to make the camp's new exercise regimen into the top weight loss infomercial in the country. Tony tries to make himself seem like someone the campers can relate to, saying that he was a fat kid when he was younger too, but his methods of motivating the campers border on psychotic.
Tony cleanses the cabins of the campers' food caches, cuts off their contact with the outside world, and installs an exercise outline of trendy fitness techniques that downplay fun to the point of humiliation.
The campers discover a secret food stash (which is restocked by other people in exchange for money at night) and actually gain weight despite Tony's fitness regimen. Eventually, at the halfway point of summer, Tony decides to weigh the boys and mark their progress. Once he realizes that the boys have been gaining weight instead of losing it, Tony forces them on a 20-mile hike, reasoning that this will not only help the boys work off some of their extra weight, but will also restore discipline. On the hike, the campers trick Tony into falling into a deep pit, severely injuring him. The boys bring Tony back to camp and imprison him in a makeshift cell of chicken wire electrified with a bug zapper. They also undo all of Tony's modifications to the camp, and incapacitate his counselors, some of whom eventually turn to their side.
In the celebration of Tony's downfall, the boys throw a bonfire party with a lot of binge eating. They order in junk food such as pizzas and submarine sandwiches, gorge themselves on chocolate and drench themselves in soda.
The next morning, Pat Finley, a counselor who had come to Camp Hope every summer since he was 10, tells the kids to finally start taking responsibility and start actually losing weight. The boys begin following a more healthy regimen and start to make Camp Hope a fun place again.
On parents' visiting day, the parents are shown a video of Tony's cruelty. While they are watching, Tony escapes his prison and ends up exchanging quips and then blows with Gerry's father. In an attempt to make an impressive exit, Tony attempts a series of backflips, stumbles, and incapacitates himself. The parents tell Tony his days of terrorizing their kids are over. Tony's own father shows up to take the keys and deed for the camp away from his son to ensure this doesn't happen again. He states that the camp will be closed, and all of the money paid for admission refunded.
But the campers don't want to leave Camp Hope. Despite Tony Perkis, the camp and the friends they have made are still a lot of fun, so Tony's father appoints Pat as the camp leader. Afterwards, Pat starts really putting the campers to work to win an annual competition against some rather athletic, and perhaps somewhat over-competitive campers who are trained to go at this competition with everything they have, which up until Pat took over made the competition rather one-sided. Pat, however, has been training them not to lose hope, and just to have fun, which they do. It turns out that they have just enough ability to win, which they do in the end, to the distress of the counselors at the overly-competitive athletic camp, who have already decided that the trophy belongs to them, and believe that Pat is crazy for being more concerned about having fun than winning. Pat has the trophy thrown into the lake and then kisses Julie, the camp's new nurse whom he'd been developing a relationship with throughout the film, in triumph as Gerry thanks Pat for the greatest summer of his life.
In a post-credits scene, Tony is shown as a door-to-door salesman selling healing crystals.
- Aaron Schwartz as Gerald "Gerry" Garner
- Ben Stiller as Tony Perkis, Jr. / Tony Perkis, Sr., Tony’s father
- Tom McGowan as Pat Finley
- Tim Blake Nelson as Roger Johnson
- Jeffrey Tambor as Maury Garner
- Joseph Wayne Miller as Salami Sam Dampier
- Jerry Stiller as Harvey Bushkin
- Anne Meara as Alice Bushkin
- Shaun Weiss as Josh Birnbaum
- Kenan Thompson as Roy Murphy
- David Bowe as Chris Donnelly
- Leah Lail as Julie Belcher
- Paul Feig as Tim Orford
- Tom Hodges as Lars
- Max Goldblatt as Phillip Grubenov
- Robert Zalkind as Simms
- Patrick La Brecque as Dawson
- Allen Covert as Kenny Parry
- Cody Burger as Cody Farley
- David Goldman as Nicholas Wales
- Judd Apatow as Homer Schulz
- Lia Lucis as Herself
- Lauren Michelle Hill as Angelic Girl-Josie