While an excavation team from the South Korean Army works diligently to uncover remains at a battlefield from the Korean War in order to establish a memorial site, an elderly man receives the haunting news that some of the remains they have identified might be those of his brother. Overwhelmed with emotion, he rushes to the site accompanied by his granddaughter. The narrative then shifts back to Seoul in June of 1950, where the Lee family navigates life under tense circumstances. Jin-Tae Lee (Jang Dong-gun) runs a humble shoeshine stand, working tirelessly to support the education of his younger brother Jin-Seok (Won Bin). Jin-Tae's fiancée, Young-shin (Lee Eun-Ju), helps out at the local noodle shop run by the Lee family.
The tranquility of their lives is shattered on June 25 when North Korea launches an invasion, plunging the nation into chaos. As fate would have it, Jin-Seok is conscripted into the army, dragging his brother Jin-Tae along as he desperately tries to plead for his sibling's freedom. The brothers find themselves quickly thrust onto the battlefield. After surviving an intense artillery bombardment, Jin-Tae learns from his commanding officer that by earning the prestigious Taeguk Cordon of the Order of Military Merit, he could secure Jin-Seok’s early return home. Driven by love and determination, Jin-Tae volunteers for a series of perilous missions, ultimately achieving the rank of Chungsa (Sergeant). Yet, Jin-Seok expresses his fears to Jin-Tae, worried that his brother's relentless pursuit for glory may cost him his life.
As the U.N. forces, led by American troops, invade South Korea from Incheon in a bid to repel the North Korean forces, battles rage, leading to significant casualties on both sides. In one such fierce encounter, Jin-Tae captures a notable North Korean captain ([Choi Min-Sik]) and receives his long-desired medal. However, this victory comes at a high cost—the loss of his close friend Yong-man, which leaves a heavy burden on Jin-Seok. With their unit advancing northward, they bear witness to horrific massacres, the grim specter of war weighing heavily on their hearts. When a patrol leads them to a group of disarmed North Korean soldiers, one of them turns out to be Yong-Seok, a boy Jin-Seok once taught. While the rest of the team desires to execute them, Jin-Seok stands firm, asserting that taking the lives of unarmed prisoners would make them no better than those who had committed atrocities before. Ultimately, they take the North Korean soldiers as POWs.
Soon, the war's landscape shifts dramatically as China intervenes, pushing U.N. forces into chaotic retreat. Amidst the chaos, Yong-Seok is fatally caught in crossfire during an encounter involving Jin-Tae. The situation escalates as Young-shin, alongside Jin-Seok, finds herself entangled with anti-communist forces. To procure food for the family during dire times, she makes a desperate choice to align with the Communist Workers' Party of Korea, which results in grave accusations against her character, plunging the family deeper into crisis. In the ensuing chaos of a failed prison escape, tragedy strikes as Young-shin is shot and killed, leaving the brothers devastated as they are arrested for attempting to rescue her. In a moment filled with agony, Jin-Seok witnesses Young-shin's lifeless body discarded where the executed prisoners lie.
As Jin-Seok endures imprisonment, he mocks Jin-Tae for his failure to protect Young-shin. But fate takes a turn when Jin-Tae enters the interrogation room; his plea for his brother’s release is met with refusal, resulting in a cataclysmic artillery strike. In the confusion, Jin-Tae loses consciousness, awakening to the false belief that he has lost Jin-Seok to the flames of the prison fire. Out of a surge of rage and despair, he brutally dispatches the security commander before being restrained by Chinese soldiers. Unbeknownst to him, Jin-Seok is alive, having narrowly escaped the inferno, thanks to a soldier known as Uncle Yang. However, Jin-Seok remains apathetic upon learning Jin-Tae's fate, eventually hearing news that his brother has defected to the North. Heartbroken, he reads Jin-Tae's heartfelt letter, igniting a spark of hope that pushes him back into the fray, rejoining the army to face the North Koreans.
Against overwhelming odds, he slips away from captivity and submits himself to the North Korean forces, claiming to be Jin-Tae's brother. Instead of recognition, the ensuing battle sees him confronted by a formidable North Korean unit led by none other than Jin-Tae himself. The brothers' reunion turns tragic, culminating in an intense conflict as Jin-Tae, still unaware of his brother's identity, seeks to eliminate Jin-Seok amidst the chaos of battle. As the fight reaches a fever pitch, something clicks for Jin-Tae as he finally sees Jin-Seok, igniting an emotional climax that neither could have anticipated. However, the tide of war is unforgiving—Jin-Tae ultimately sacrifices himself to cover for the South Korean retreat, his final moments filled with pride and sorrow for the brother he fought so hard to protect.
In the present narrative, the aged Jin-Seok returns to the excavation site, where he gazes in anguish at personal items of Jin-Tae, including the silver pen filled with promise and nostalgia. Overwhelmed by memories and a desire for connection, he begs his brother's remains to speak to him, recalling the solemn promises made against the backdrop of the war-torn landscape. With his granddaughter looking on, Jin-Seok finds a haunting sense of irony in how this precious item now lies in his possession—a symbol of their bond and the promise of a future. The film closes in a poignant moment, revealing the aftermath of the Korean War, as Jin-Seok returns home to his mother alongside Young-shin's siblings, reassuring them of his return to education, thereby fulfilling Jin-Tae’s cherished promise.
