After two days the guards turn violent and dehumanizing towards the inmates. The experiment is never stopped. After six days, a full scale riot breaks out, and the experiment ends. Benjy dies inside, and the lead researcher is convicted of manslaughter. The experiment's purpose is never explained.In the end, Travis, for no explained reason, moves to India with the girl he met only days ago.
26 men sign up for a scientific experiment. All they are told before hand is that some will be stripped of all civil rights, some will not.
When they arrive at the compound, it is revealed that 8 of them will become prison guards, and the other 18 will be inmates and this will last for 14 days. All of the subjects are warned that if there is any physical violence that the experiment will be shut down and everyone will forfeit all of their money (14,000$ for two weeks). The guards are also warned that if an inmate breaks any of the rules enforced on them (clean their plates, don't talk unless spoken to, exercise for 30 minutes a day, never touch a guard, nothing from the outside world is allowed in) and are not punished for it within 30 minutes, the experiment will end and all money will be forfeit.
Almost immediately, the "don't touch a guard" rule is broken, and an inmate touches the guard. The experiment is not ended, even after the 30 minutes pass.
Within two days, the guards are doing horrifically cruel things to the inmates. Fearing that they will lose their 14,000$ pay check if there is a riot among the prisoners, they tie Travis (Adrien Brody) to a chair, shave his head, kick him to the ground, then several guards urinate on his face. Despite warnings that if anything violent happens the experiment will end, nothing happens and nobody stops them.
In addition, one of the inmates lied on his application. He has diabetes, and needs insulin. Within two days, he's beginning to die, but the experiment is not called off and the guards cannot give him the insulin because of the "nothing from outside world is allowed in" rule.
Day four, Travis bribes a guard into trying to steal the insulin from the bag to save the inmate. He does it, but is caught. The guard leader, Barris (Forest Whitaker), allows the dying inmate to have the insulin (once again breaking the rules, and once again nothing happens). They take Travis into the bathroom and constantly dunk his head in the toilet as punishment for trying to bribe a guard.
Day five, the guard that tried to get the insulin has a bag thrown over his head by the other guards and is beaten with batons. Yet again, a rule is broken, and nothing happens. They drag the beaten guard out as an example, showing the inmates that they can use violence and nothing will happen. In addition, he is stripped of his guard status and is made into a prisoner.
The inmates begin to become insubordinate, all of them stripping out of their prisoner uniforms at the same time. Travis yells to a camera that he is finished, but nothing happens. Barris pulls him from the fence and slams him to the ground. Benjy punches Barris in the back of the head, and he responds by clubbing Benjy over the head with the baton, cracking his skull and causing him to begin having a seizure.
All of the inmates are separated and locked in different rooms.
Day six, Travis breaks out of his cell. He finds a guard about to rape am inmate and they knock him out and steal the key. One by one, then run and unlock all the prisoners from their cells. They find Benjy dead.
A full-scale riot starts. Using everything they can get, the inmates begin to break down the door to the guard's office. All of the remaining guards tell Barris they are done and begin to run for an exit. Barris follows.
All of the inmates catch up with the guards, and begin to beat them. Finally, the alarms go off, signaling the end of the experiment.
All of the fighting immediately stops as the gates are opened, and everyone still alive walks outside and waits for the bus. They are given their $14,000 checks on the bus.
The end of the movie shows that there is a news report on the test. The company had "loose ties to the government," and Travis was caught on camera saying that they would testify in court. A voice over says that the lead researcher on the project was arrested and charged with manslaughter for the death of Benjy. What the point of the whole experiment was is never revealed at all.
The ending of the movie shows that Travis, for reasons unexplained, has moved to India with the girl he met only days ago at the peace rally.