Robert is an American university Spanish professor volunteering for the Republican army as a demolition specialist. Operating behind enemy lines, he approaches his assignments with a strict, emotionless dedication to duty. When ordered to destroy a strategic bridge in a specific timeframe, he travels into the mountains, where he must rely on unpredictable guerrilla fighters and confront his own rules about remaining detached from the people he works alongside.

Pilar is the wife of the guerrilla leader Pablo and a formidable presence in the camp. Recognizing Pablo's growing weariness and reluctance to fight, she naturally commands the loyalty of the other fighters, effectively taking control of the group. She is deeply committed to the Republican cause, highly perceptive, and acts as a fiercely protective maternal figure for Maria.

Pablo is the original leader of the local guerrilla fighters. Early in the war, he was a ruthless and effective commander, but the prolonged violence has left him weary, cynical, and highly resistant to risking his life further. His strong opposition to Robert's mission creates an immediate fracture within his own camp, as the other fighters begin to view his caution as cowardice.

Maria is a young woman recovering in the guerrilla camp after surviving severe abuse and the execution of her parents by fascist soldiers. Pilar orchestrated her rescue from a prisoner train and oversees her daily well-being. Maria's sudden connection with Robert offers her a path toward healing and gives her a reason to look toward the future.

Anselmo is an elderly, experienced Spanish guerrilla fighter who acts as Robert's initial guide. Despite participating actively in the war, he maintains a deep hatred for killing and struggles morally with the violence required of him. He provides Robert with unwavering loyalty and serves as a compassionate, grounding force in the group.

El Sordo is the capable and respected leader of a nearby Republican guerrilla band. He operates in the same mountain range as Pablo and Pilar, maintaining a well-hidden camp. He understands the extreme danger of Robert's proposed demolition but agrees to supply manpower out of pure dedication to the cause.

Agustín is a fierce and highly vocal member of Pablo's guerrilla band. He harbors an intense hatred for the fascists and often speaks aggressively about his desire for violence against them. Underneath his coarse exterior, he is deeply protective of Maria and highly loyal to Pilar's leadership.

Rafael is a member of the guerrilla band who sometimes struggles with the discipline required for their survival. He is easily distracted, famously abandoning his guard post to hunt for fat rabbits in the snow, which earns him a severe reprimand from Robert.

Fernando is a pragmatic and literal-minded member of the guerrilla band. He frequently travels to nearby towns to gather information, bringing back rumors of troop movements and Republican offensives that help inform the group's tactical decisions.

Primitivo is a member of Pablo's band who possesses a strong sense of solidarity with his fellow fighters. When a neighboring camp falls under attack, he struggles immensely with Robert's logical orders to remain hidden rather than rushing blindly to assist their allies.

Andrés is a member of the guerrilla band assigned the highly urgent and dangerous task of carrying Robert's written message through enemy-contested territory to General Golz. He faces constant bureaucratic and physical delays on his journey.

Lieutenant Berrendo is a Nationalist cavalry commander forced to take charge after his superior is killed. He represents the enemy forces closing in on the mountains. Though he fights for the fascists, he privately despises the extreme brutality of the war and the gruesome orders he is expected to carry out to please his leaders.

General Golz is the high-ranking Republican officer who assigns Robert his mission. He orchestrates the broader military offensive that requires the bridge to be destroyed at an exact time, operating far from the immediate physical dangers of the mountain camps.

Joaquín is a young guard in El Sordo's camp. He carries immense grief, having lost most of his family members to fascist violence. His emotional trauma serves as a harsh reminder to Robert of the real human cost of the war.

Karkov is an influential Russian figure and journalist based in Madrid. He possesses a deep understanding of the political machinery driving the war and the rumors that circulate through the city. Robert thinks of him often when considering the broader implications of the conflict.

Gómez is a soldier who guides and assists Andrés through the maze of Republican camps and checkpoints, using threats and persistence to cut through the military bureaucracy.

André Marty is a formerly great French military leader whose paranoia and corruption now hinder the Republican cause. He creates severe obstacles for the delivery of vital field communications by needlessly detaining messengers.

Duval is a commander operating at General Golz's headquarters. He acts as an administrative bridge between the frantic messengers arriving from the field and the high command executing the battle plans.