The Human Condition follows the journey of the well-intentioned, yet naive Kaji who transitions from being a labor camp supervisor to an Imperial Army soldier and eventual Soviet POW. Constantly trying to rise above a corrupt system, Kaji time and time again finds his own morals an impediment rather than an advantage.

In the second film, Road to Eternity (1959), Kaji, having lost his exemption from military service by protecting Chinese prisoners from unjust punishment, has now been conscripted into the Japanese Kwantung Army. Under suspicion of leftist sympathies, Kaji is assigned the toughest duties in his military recruiting class despite his excellent marksmanship and strong barracks discipline. His wife Michiko pleads for understanding in a letter to his commanding officer and later pays Kaji a highly unorthodox visit at his military facility to express her love and solidarity. Kaji considers escape across the front with his friend Shinjo, who is similarly under suspicion due to his brother's arrest for communist activities. Distrusting the idea that desertion will lead to freedom, and faithful to his wife, Kaji ultimately commits to continued military service despite his hardships.

When Obara, a poor-sighted, weak soldier in Kaji's unit, kills himself after troubles from home are compounded by ceaseless punishment and humiliation from other soldiers, Kaji demands disciplinary action from his superiors for PFC Yoshida, the ring leader of the troops who pushed Obara over the brink. While Yoshida is not disciplined, Kaji helps to seal his fate by refusing to rescue the vicious soldier when both men are trapped in quicksand while in pursuit of Shinjo, who finally seized the opportunity to desert. Part three of the trilogy (and the first half of this film) ends as Kaji is released from hospitalization related to the quicksand incident and is transported to the front with his unit.

As part four opens, Kaji is asked to lead a group of new recruits and promoted to private first class. He accepts his assignment with the condition that his men will be separated from a group of veteran artillerymen, who practice intense cruelty as punishment for the slightest offenses. Often taking the punishment for his men, Kaji is personally beaten many times by these veterans, despite his personal relationship with Second Lieutenant Kageyama. Demoralized by the fall of Okajima and continually battling with the veterans, Kaji and most of his men are sent on a month-long trench digging work detail. Their work is interrupted by a Soviet army onslaught that produces heavy Japanese casualties and the death of Kageyama. Forced to defend flat terrain with little fortification and light armament, the Japanese troops are overrun by Soviet tanks, and untold men are killed. Kaji survives the battle, but is forced to kill a maddened Japanese soldier with his bare hands in order to prevent Soviet soldiers from discovering his position. The film ends with Kaji screaming, "I'm a monster, but I'm still alive!" and running in desperate search of any other Japanese survivors.