Set during the Algerian War, The Battle of Algiers opens in 1957. A group of French paratroopers surrounds a National Liberation Front (FLN) member who they have just broken through interrogation. They dress the shaken-looking, emaciated Arab Algerian in a French military uniform before making him lead them to the hiding place of Ali La Pointe, an FLN leader. With three other FLN members, Ali squats in his hideout behind a section of wall. Ali listens as the French promise him a fair trial if he surrenders.
The film cuts to 1954. After being arrested for running an informal gambling table on the street, Ali—an illiterate laborer and boxer—is imprisoned. While in prison, Ali witnesses the brutal guillotine execution of an Arab man who shouts Long Live Algeria! on his way to his execution.
Five months later, Ali, now free, returns to the Casbah—the traditional Islamic quarter of Algiers. A boy gives Ali instructions passed from an FLN member to assassinate a French police officer. The plan involves receiving a pistol from a woman dressed in a white burka. When Ali takes the shot, however, the gun has no bullets. Ali chases down the woman and demands to know who set him up. The woman then takes Ali to meet Djafar, an FLN member who explains that they had to make sure Ali wasn't a spy. Jaffer explains the need for the FLN to make the Casbah a safer place for them to use as a hideout and a base to work from, as there are too many people willing to sell them out.
In 1956, the FLN declares all vices forbidden. With newfound power, Ali assassinates an Arab man who refuses to join the FLN. After shooting the man, Ali announces that no one can do as they want in the Casbah anymore. That July, the FLN coordinates a series of simultaneous assassinations of French policemen in Algiers. In response, France increases its police presence and sets up checkpoints around the Casbah, searching anyone who comes out. However, they don't search women in burkas out of respect for Muslim tradition; the FLN exploits this by having women sneak out pistols that men use to shoot police.
One night, a French police chief enters the Casbah at night with several subordinates. In retaliation for the assassinations, the police set off a bomb that destroys a building, killing civilians that include women and children. The people of the Casbah gather in a march and set out for revenge. However, Jaffer intervenes to assure them that the FLN will get revenge for them; otherwise the police will kill them all.
Three female FLN members shed their burkas, style their hair and apply makeup. They put on European clothing that makes them look like they are members of the secular community. Once transformed, they leave the Casbah without being searched or questioned and proceed to plant time bombs (concealed in purses) in cafes and bars full of young French settlers, as well as an airport lounge. The bombs explode in succession, killing many civilians and stretching emergency response workers over the three sites.
In 1957, Lieutenant-Colonel Mathieu Philippe comes to Algiers to plot a counterterrorism offensive against the FLN. Because the insurgent rebel group has a triangulated structure ensuring that each member only knows three others, Philippe plans to use interrogation to make connections and hunt down the chiefs of the FLN.
The FLN coordinates a mass strike of Algerian workers in an effort to get the UN's attention and show the international community that the FLN represents more than a small minority of Algerians, and that the masses want independence from France. Ali, who does not like the idea, meets with Ben M'Hidi, the FLN's leader. He defends the idea of the strike and says the hardest thing about a revolution is not to start it but to continue it.
A few days into the strike, Philippe orders his men to round up Algerian men from the Casbah so that he may interrogate them. They use flames, ropes, and waterboarding as tactics. Eventually, he connects enough information to learn the identities of the chiefs of the FLN, and distributes photos of the men to his soldiers. Philippe also begins a 24-hour-a-day occupation of the Casbah, which involves announcing over a loudspeaker that the FLN has fallen.
Meanwhile, the FLN chiefs go into hiding within the Casbah. Hiding spots include secret cavities behind walls, water wells, and under burkas when having to move between places. Even though they are under supervision, the FLN plants a bomb at a horse race.
At a press conference, Philippe shows off his capture of Ben M'Hidi, who takes questions from reporters. He smugly defends the FLN's tactics, explaining that they are minor atrocities compared to those carried out by hegemonic powers like France and the U.S.
At another press conference some time later, reporters question Philippe about M'Hidi hanging himself in his cell. Based on the questions, it is clear the press has been reporting unfavorably about Philippe's torturous interrogation tactics. Philippe insists that the tactics are necessary if people want the country to remain a French colony.
Philippe continues to brutally torture Algerians to collect information about the FLN heads. The tactics are successful in that he is able to hunt down everyone, including Jaffer, who agrees to surrender following a standoff.
The last FLN head is Ali La Pointe. The film returns to the opening scene in which a broken FLN member gives away Ali's hiding place. Philippe attempts to negotiate with him through the section of wall, but Ali refuses to speak with him. The negotiation ends with Ali and his three FLN companions dying in a large explosion the French set off. Philippe walks away from the bombed-out site, pleased that he has killed the head of the FLN.
The film ends with a scene set in 1960. The streets of Algiers are filled with Arab Algerians waving Algerian flags and crying out against French colonial rule. The crowds push back the police, who are outnumbered. The camera lingers on the faces of women who lead the wave of protesters. A voiceover explains that two more years of struggle passed before Algerians won their independence from France in 1962.
