In 2022, the penal system is run by corporations, and prisoners are treated as assets. Casey is a highly-trained ex-Marine imprisoned for life for killing his commanding officer after being ordered to kill innocent civilians in Benghazi; after escaping from two Level 5 maximum-security prisons, he is sent to a Level 6 facility. A fellow prisoner tells him about "Absolom," an island where the worst prisoners are sent and is feared more than the prison. The Warden Michael Lerner, after being held at gunpoint by Casey, exiles him to the island.
On the island, Casey is captured by a group called the Outsiders, led by the sociopath Walter Marek Stuart Wilson, who rules with The Law of the Jungle. The abandoned island resort, with dilapidated houses and a pool, serves as their base. Marek makes Casey fight one of his men for amusement and is impressed when he kills the opponent quickly. He offers him a spot in the gang, but Casey knocks Marek into the pool, steals his rocket launcher, and escapes the Outsiders' camp. He is chased through the jungle and cornered at a cliff's edge, then shot in the neck by blowgun darts. After falling into the river below, he is retrieved by another prisoner group—the Insiders.
Led by a terminally ill doctor known as the Father Lance Henriksen, the Insiders have built a cooperative autonomous community with laws as opposed to the tyranny of the Outsiders. Casey meets the Father, Hawkins Ernie Hudson, the head of security; King Ian McNeice, a hypochondriac new-arrival helper; and Casey, a naïve young man given a life sentence for his minor role in a kidnapping which led to a dual felony murder when the hostages accidentally died. Robbins learns that the Insiders are heavily outnumbered by the Outsiders and that he is the only person to have escaped Marek's camp. The weapon he stole is appropriated for the community.
After helping repel an attack by the Outsiders, Father notes Robbins' abilities and asks him to join them. He refuses, saying he wants to leave Absolom. Father takes Robbins to the shore, explaining that they are 200 miles (321.9 km) from the mainland. Gunships patrol 50 miles (80.5 km) off the coast, and infrared satellite technology monitors thermal activity, such as large fires or explosions.
The Insiders have secretly built a scan-proof boat and launch it to tell the outside world about Absolom. However, the boat is destroyed by attack helicopters, causing Father to believe there is an informer among them.
Robbins discovers a new boat is being built and demands a seat, still determined to escape. After learning that the engine needs a distributor, a part Robbins saw while in Marek's camp, he offers to retrieve it in exchange for passage and the Insiders agree. Casey follows Robbins, who infiltrates the Outsiders' camp and gets the engine part, but they are captured by the Outsiders. Robbins is forced to fight Casey to the death. Realizing they won't both escape, Casey impales himself on Robbins' weapon. Robbins escapes a planned execution with the help of an Insider spy, infuriating Marek.
Knowing Marek will attack, and over stern objections, Robbins convinces the Insiders to abandon their camp, but not before lacing it with booby traps. The Father attempts to convince Robbins to be the leader of the group when he dies, as he secretly has been suffering from progressive Hodgkin lymphoma that he estimates could kill him in a month without any sort of proper treatment. He tries to recruit Robbins to take his place as leader, but Robbins asserts his need to escape the island before coming to accept that the need to assert the truth of what really happened that led to his imprisonment and the truth about the island are equally important. When the Outsiders attack the abandoned camp, Robbins stays behind to fire the stolen rocket launcher, igniting an incendiary bomb. Most of the Outsiders die, and the Warden intervenes when the satellites are triggered.
The Father dies while defending Robbins from Marek. After Robbins kills Marek, he finds the new boat incinerated and evidence that King killed the engineer. Robbins attacks King, who confesses to acting as a spy for the Warden in return for his freedom. Robbins forces King to give new coordinates to the Warden for where to land his helicopter on the island. Robbins tells Hawkins that The Father told him that Hawkins should be the new leader before enacting a plan with the helicopter. With a group of fellow Insiders, Robbins hijacks the helicopter, throws the Warden out, and vows to spread the truth. King and the Warden are left behind as unseen Outsiders close in on them.
