When summer break begins, Gerry Garner is informed by his parents that they are sending him to Camp Hope, a weight loss camp specifically for boys. Although Gerry is initially hesitant about this decision, he soon encounters the spirited camp counselor Pat and develops friendships with fellow campers who have managed to sneak in copious amounts of junk food, enough to last the entire summer. On the first night at Camp Hope, a shocking announcement reveals that the original owners, the Bushkins, have gone bankrupt, and the camp has been taken over by fitness mogul Tony Perkis Jr.. His vision is to pivot the camp's weight loss focus into a lucrative infomercial.
Under Tony's direction, the campers' favorite activities, like go-kart racing and the fun-filled “Blob,” are scrapped in favor of grueling exercise routines. The beloved counselor Pat is suddenly replaced by the strict Lars. The campers must endure an excruciating softball match against their rivals, Camp MVP, who are far more athletic and competitive. When Tony discovers hidden food supplies in the cabins and decides to eliminate them, the bonds of friendship among the campers strengthen, especially when camper Josh stands up for Gerry by mocking Tony, resulting in Tony sending him home without a refund.
In a bid to make the boys lose weight, Tony arranges a humiliating dance with the girls from nearby Camp Magnolia. However, counselors Tim, Pat, and camp nurse Julie encourage everyone to have fun instead. Josh makes a return to Camp Hope and reveals that his lawyer father has threatened legal action against Tony for expelling him without a refund.
Determined to retrieve their stolen snacks, Gerry and his friends break into Tony’s office and stumble upon an unsettling truth: Tony has been intercepting all the campers’ letters to their families, including one Gerry had sent to his grandmother. In their quest, they discover a hidden stash of food that many campers had been using, leading them to gain weight despite Tony's harsh regimen. To assert control, the boys devise a plan that sees Tony fall into a pit and become trapped in a makeshift cell, where he is subjected to the shocking buzz of a bug zapper. With the support of Pat, Julie, and Tim, the campers reclaim Camp Hope, subduing Lars and Tony's other counselors and celebrating a boisterous binge-eating bonfire party. Ultimately, Lars sides with the campers, fearing deportation.
The next day, Pat inspires the campers to take charge of their own lives and adopt healthier habits, which rekindles the camp's spirit. When the campers’ parents arrive for visiting day, they are shown a video highlighting Tony's cruel treatment. Just as the screening is interrupted by a re-emerged Tony, tensions bubble over as he confronts Gerry's father, leading to a chaotic encounter where Tony’s attempt to escape fails spectacularly.
As Tony is taken away, his father arrives to promise refunds, but he reveals that the camp will close down. However, the boys rally to keep Camp Hope open. With Pat stepping up after 18 years of experience, the camp is set for a fresh start with support from Gerry and his friends.
Under Pat’s guidance, the campers restore their beloved activities and gear up for their annual competition against Camp MVP. The events kick off with an obstacle course where Camp MVP leads. However, in the knowledge challenge that follows, Camp Hope catches up, and in the thrilling conclusion of the go-kart race, Gerry emerges victorious for Camp Hope. Recognizing that enjoyment surpasses the significance of winning, Pat tosses the trophy into the lake and seals his romance with Julie with a kiss. As Camp Hope revels in their victory, Gerry expresses his gratitude to Pat, declaring it the best summer he has ever had.
During the credits, Pat, the campers, and Lars joyously perform the "Camp Hope" song together.
In a humorous post-credits scene, Tony has hit rock bottom as a failed door-to-door salesman, now hawking dubious healing crystals.
