Agu is a cheerful pre-teen boy playing barefoot with his friends in an African village. He lives in a Christian home, a modest but clean family dwelling with a father, mother, teenage brother, senile grandfather and infant sister. There is talk of war, sounds of gunfire in the distance and UN troops but the family has high spirits and laugh and joke around. As the civil war gets closer the villagers decide to send the women and children to the Capital for safety. In a melee, there is no room for Agu as the packed cars and taxis depart.
A short time later the government troops enter the village. The remaining men and boys try to hide but are discovered, an angry Sergeant declares them as spies and orders them killed. The prisoners scatter but are shot, Agu's brother and father are killed but he makes it out to the forest in safety.
Agu, wandering in the bush getting hungry is captured by a gang of armed boys, they have machetes, AK-47s and other weapons. The leader is a man called Commandant (Idris Elba) who lets Agu join their battalion. First he is a carrier, hauling an ammo box. The battalion hikes to their camp high in the hills. There Agu is further indoctrinated and brainwashed into the group, and receives training in small arms platoon combat. There is another boy is same age named Strika, angry who never talks. An earnest young man is the second in command referred to as "2-I-C".
The first real mission is a road ambush. The youths hide around a bridge and attack a convoy of jeeps and trucks. Successful, they strip the dead enemy of anything valuable. A survivor is brought to the Commandant, who then gives Agu a machete and orders him to kill the man. Pausing and needing some prodding, Agu finally hacks he pleading man, Strika joins in and they chop away furiously. The battalion take the vehicles and drive off.
In a town the battalion attacks as part of a larger force to take over a bridge. The Commandant personally leads his troops by coaching and exhorting the scared teens in his unit in a real combat situation. At night they relax and the Commandant calls in Agu to his office. He is on the phone with the Supreme Commander getting congratulations and his next mission. Agu has found a tinfoil pack of drugs on a dead body and offers it to the Commandant. The Commandant teaches Agu the importance of obedience in the military and following orders even if you don't like them. He encourages Agu by telling him he has leadership potential. Going into the bedroom the Commandant snorts the drugs, lays back on his cot and summons Agu to pleasure him.
Leaving the hut, Agu is stunned and quiet, Strika comforts him.
The battalion attacks another town, shooting people on the streets and ransacking houses. Agu and his team find a woman and young child hiding in a bedroom. Confused, Agu thinks the woman is his mother and tries to hug her, then realizes she is not. Upset, he paces around then shoots her in the head as another teen is starting to rape her. The boys then stomp the infant. After, Agu starts using drugs like the older boys in the battalion.
The Commandant is summoned to see the Supreme Commandant, he brings Agu, Strika and a few others. They are made to wait several hours before the elegant politician deigns to see him. The white suited man announces the Commandant will be promoted to Deputy Security Chief, but the Commandant sees it as a demotion and wonders about his pay.
For his last night with his boys the Commandant takes them to a brothel. The older ones go to rooms with the ladies while Agu and Strika drink beer and sleep in the reception area. A gunshot rings out and the 2-I-C staggers out clutching his stomach. A prostitute says it was an accident as she was playing with his gun. Frantic, the Commandant orders the women shot and the men leave quickly. 2-I-C dies later.
The Commandant refuses his promotion and takes his battalion back into the bush. In a skirmish Strika is shot, Agu tries to carry him piggyback but the boys dies. They cover him with leaves and continue. In a fetid en-trenched base where are the troops are either drugged up or shooting randomly, they approach a weary Commandant. The new 2-I-C says they have no money or bullets and are leaving. Everyone points weapons at each other but the Commandant relents and let's them go. A short while later a UN force disarms and captures the straggling teens.
Agu is sent to a pleasant ocean-side camp with other youngsters. The boys play soccer and swim in the surf and receive counselling. A reticent Agu goes through the DTs to get over his drug addiction and finally opens up to a social worker and goes to play with the others.