In 1943, as the Allied forces prepare to strike at North Africa, Algiers sits under the shadow of the Vichy government and the German Armistice Commission, setting the stage for a mission whose success could spare countless lives. Alan Thurston, a British agent, is dispatched to locate a camera that holds photos pinpointing where a pivotal Allied conference will be held, a clue that could steer the invasion with minimal casualties. He moves carefully through a city tense with occupation, aware that he is being tracked by the enemy.
The trail leads to a dangerous confrontation with the German spy Dr. Müller, who is bent on stealing the camera as soon as Thurston secures it. But before the camera can fall into German hands, Thurston is captured by the Vichy French and kept under guard in a fort, where he makes a bold escape. As he darts to freedom, the urgency of his mission pushes him into a tense, life-or-death chase across occupied territory. He finds reluctant shelter in the home of an American sculptor, Susan Foster, in the desert town of Biskra, where her initial reaction is suspicion and danger because she mistakes the intruder for a burglar.
Susan’s wariness begins to soften as Thurston explains his predicament and pleads for help, appealing to a sense of patriotism that burns brighter than any fear. She becomes entangled in a high-stakes web of surveillance as Müller intensifies his pursuit, warning Susan to stay out of politics while Thurston explains the stakes and the camera’s significance. The mission takes a twist when Thurston asks Susan to aid in retrieving the camera, which a deceased agent hid in a bureau at the agent’s lover’s home, the nightclub singer Maritza, who is blissfully unaware of the danger surrounding her.
In a bid to verify the truth, Susan steals the camera from Maritza’s bedroom in Algiers. Her initial plan to deliver it to the American consulate is complicated by suspicion and pressure, especially when Dr. Müller himself detains her, and a fellow ally, Henri de Lange, a Vichy officer, proves unable to withstand German intimidation. Thurston arrives to rescue her, their partnership deepening even as danger closes in from every side.
The pair take refuge in a kasbah with Yvette, who reveals a more complex side to the plot, confessing that she lied about Thurston’s honesty out of jealousy to deter Susan from misguided choices. With the camera’s contents still unrecovered, Schultz, Müller's conspiratorial ally, locates Thurston and demands the device. Thurston hands over the camera, which now contains no film, but Susan’s quick thinking allows them to gain the upper hand: she drops down and tackles Schultz, giving Thurston the opportunity to incapacitate him.
A tense pursuit ensues, with Schultz in hot pursuit, but Yvette jeopardizes herself to buy them more time by shooting Schultz. The escape continues as they commandeer Müller’s car as a decoy to draw German attention away from the critical source, while they race to warn Allied officers of potential exposure. The plan works in the larger sense: the invasion proceeds with the Allies landing as scheduled, and the danger begins to recede, even as the two protagonists’ bond deepens.
The climax resolves with a poignant farewell as they part ways for a moment, Susan being dropped off before a fateful cliffside crash of the decoy vehicle. In the aftermath of victory, Susan recounts the perilous tale to a nurse, a tale that culminates in a dramatic reappearance: Thurston steps into the room, and the two share a long-awaited embrace, the tension finally giving way to a quiet, mutually earned relief.
