The plot of "Tae Guk Gi" is driven by a series of tragic twists that escalate the conflict between the brothers. The first major turn is Jin-tae's transformation from a caring brother into a ruthless war machine. This change, which Jin-seok witnesses with growing horror, creates the central rift between them. A critical turning point occurs when Jin-tae's fiancée, Young-shin, is executed as a suspected communist sympathizer by a zealous South Korean militia. As the brothers try to intervene, they are arrested by their own side.
The most significant spoiler is the sequence of events that leads to Jin-tae's defection. While imprisoned, the building is set on fire during an enemy attack. A commander prevents Jin-tae from saving his brother, and Jin-tae wakes up believing Jin-seok perished in the flames. Consumed by grief and a profound sense of betrayal by his country, he kills the commander and subsequently joins the North Korean army, becoming the feared leader of an elite unit. In reality, Jin-seok survived and was sent to a military hospital.
The climax reveals this misunderstanding. Jin-seok, having learned of his brother's defection, intentionally seeks him out on the battlefield. They have a violent confrontation where the brainwashed Jin-tae initially doesn't recognize his own brother and tries to kill him. Jin-seok's desperate pleas finally break through, and Jin-tae has a moment of lucid recognition. To save his wounded brother from advancing North Korean and Chinese forces, Jin-tae makes his final sacrifice. He stays behind, manning a machine gun to cover Jin-seok's retreat, and is killed. The film's ending frames this entire story as a flashback. An elderly Jin-seok is at a modern-day excavation site, where the remains found have been identified as his brother's. He is given his brother's few possessions, including the fountain pen he had lost 50 years prior, and collapses in grief, finally able to mourn.