In July 2003, twelve-year-old Ben arrives at the Tom Lerner Water Polo Summer Camp. Still reeling from the emotional fallout of his parents' separation, Ben feels insecure and desperate to make friends. He quickly notices the strict pecking order established by the older and more athletic campers, led by a charismatic but ruthless boy named Jake. To protect himself from being targeted, Ben attaches himself to Jake's circle. The group focuses their daily torment on Eli, an awkward, quiet boy with red, flaky patches of eczema on his arms and neck. The boys brand Eli's condition The Plague, claiming that anyone who touches him will catch a brain-rotting illness. They isolate Eli at lunch, throw his belongings into the swimming pool, and humiliate him during training drills while Coach Daddy Wags dismisses their behavior as harmless boyish bonding. One evening, Ben stumbles upon Eli secretly weeping in the camp bathhouse as he tries to apply medicated cream to hard-to-reach sores on his back. In an unexpected moment of empathy, Ben takes the tube and quietly helps Eli spread the lotion. The two boys share a brief, silent understanding. However, the next morning, guilt and terror take hold of Ben. After noticing small red bumps appearing on his own forearms, Ben becomes convinced that Eli's disease is real and contagious. As the boys plan a final, extreme prank against Eli, Ben must choose whether to step forward and confess his secret or watch his peers cross an irreversible line.

During the final weekend tournament, the campers participate in a nighttime ritual in the woods surrounding the camp. Jake and the other boys ambush Eli, intending to tie him to a tree and cover him in mud to cleanse him of the plague. Ben stands among the attackers, paralyzed by fear and frantic over the growing rash on his own chest, which is revealed to be a severe case of stress-induced hives caused by extreme guilt and panic. When Eli looks directly at Ben and whispers that he thought they were friends, Ben finally breaks. Instead of joining the assault, Ben steps between Jake and Eli. He screams at the boys that the plague is just regular eczema and peels off his shirt to reveal his own stress hives, forcing the group to face their irrational fear. Jake hesitates, realizing that the illusion of safety within the mob has shattered. Coach Daddy Wags arrives after hearing the commotion, breaking up the gathering and finally seeing the severity of the bullying. In the final scene, parents arrive on the last day of camp to pick up their children. Jake and his friends sit quietly on their duffel bags, stripped of their power and avoiding eye contact. Ben helps Eli carry his luggage to the parking lot. Though they do not speak, Eli gives Ben a gentle nod of gratitude before getting into his mother's car. Ben climbs into his mother's station wagon, scarred and exhausted, but carrying a newfound sense of personal integrity and independence.