Agu is an innocent pre-teen boy who delights in playing barefoot with his friends in a quaint African village. Living in a loving Christian household, his family consists of his father, mother, a teenage brother, a senile grandfather, and an infant sister. Despite the ominous talk of war and distant sounds of gunfire, the family's spirits remain high, filled with laughter and joy. However, as the civil war approaches, the villagers decide to evacuate the women and children to the capital for safety. Unfortunately, during the chaos, Agu finds himself left behind as the vehicles depart, leaving him alone.
Shortly thereafter, government troops invade the village, leading to chaos. The remaining men and boys attempt to hide but are swiftly discovered. An enraged Sergeant accuses them of being spies and orders their execution. Amid the ensuing mayhem, Agu witnesses the tragic loss of his father and brother, managing to escape into the dense forest.
As he wanders through the wilderness, hunger overtakes him, and he is captured by a gang of armed boys, wielding machetes and AK-47s. Their leader, known as Commandant (Idris Elba), takes Agu under his wing, allowing him to join their battalion. Initially, he serves as a carrier, tasked with hauling an ammo box as they trek to their camp hidden in the hills. Here, Agu undergoes brutal indoctrination, learning combat skills alongside another boy named Strika, who remains silent and brooding. They are led by an earnest young man referred to as the 2-I-C.
Their first significant mission involves ambushing a convoy at a bridge. The boys successfully execute their attack, stripping the deceased of their valuables. When a survivor is presented to the Commandant, Agu is given a machete and ordered to kill the helpless man. Hesitant at first, Agu ultimately succumbs to the pressure and brutally murders the man, joined by Strika as they engage in the horrific act.
Throughout their operation in another town, the battalion engages in more violence, with the Commandant keenly leading his terrified adolescent troops. One night, he summons Agu into his office to discuss an upcoming mission. After discovering drugs on a dead body, Agu offers them to the Commandant, who seizes the opportunity to instruct Agu on the importance of following orders and recognizing his potential for leadership. In a disturbing turn, the Commandant then summons Agu to please him, leaving the boy shocked and confused, with Strika offering comfort afterward.
During another brutal assault, Agu reluctantly finds himself involved in the harrowing act of violence as he confronts a woman and a child, mistaking her for his mother before ultimately shooting her. Consumed by guilt, Agu begins using drugs to numb the traumatic experiences around him.
Later, the Commandant is summoned to meet with the Supreme Commandant, revealing a promotion to Deputy Security Chief – a supposed honor that the Commandant sees as a demotion, fixated on monetary compensation instead. For their final night together, the Commandant treats the boys to a chaotic night at a brothel, which ends in tragedy as one of their own, the 2-I-C, is shot, leading to an urgent escape.
Refusing the promotion, the Commandant takes the battalion deeper into the bush, where Strika is gravely injured. Agu's desperate attempts to carry his friend fail as Strika dies, and the group mourns briefly before covering him with leaves. As they stumble across a drugged-out camp, they face a tense standoff and ultimately decide to leave.
Their struggle comes to an end when a UN force intervenes, disarming and capturing the disheveled teens. Agu is then sent to a healing camp by the ocean, where he slowly starts to recover. Surrounded by other children, he plays soccer and swims in the surf, receiving much-needed counseling. As Agu navigates his way through withdrawal, he gradually opens up to a social worker, allowing himself to start anew amidst the camaraderie of the other boys.
