Sixty years after the end of World War II, Japanese researchers arrive on the Japanese island of Iwo Jima to explore the caves and tunnels built by their soldiers to defend against American attack forces. In June 1944, Imperial army commander Lt. Gen. Tadamichi Kuribayashi and his aide, Lt. Fujita, arrive at their new post in Iwo Jima to prepare defenses for the island’s crucial airplane landing strips. Upon making a walking tour of the island, Kuribayashi calls an immediate halt to the digging of trenches along the beaches, a traditional defense measure, which distresses navy and army officers alike, including dysentery-plagued Admiral Ohsugi, dedicated Lt. Ito, traditional Col. Adachi and rigid Maj. Gen. Hayashi. Kuribayashi, who years earlier served as a military envoy in the United States, recognizes that America’s great industrial strength gives their forces an enormous advantage, but is nevertheless determined to mount an aggressive defense of the island. Young Pvt. Saigo appreciates Kuribayashi’s thoughtful treatment of the soldiers, but his friend Nozaki wonders about their fate upon hearing the rumor that their new commander is pro-American and the military's second choice for the island posting. Over the next several weeks, Kuribayashi struggles to unify the army and naval forces on the island and continues to confound his officers with unusual defense plans, including moving heavy artillery from the beaches to emplacements dug into the rocky terrain of the dormant volcano Mount Suribachi at the southern tip of the island. Despite the infrequency of mail pick-up ships stopping at the island, Kuribayashi writes to his wife and children on a regular basis, describing a sanguine situation and decorating his letters with idyllic drawings. Saigo, who also writes continually to his wife Hanako and their baby, whom he has never seen, relates the arrival of nobleman baron Lt. Col. Takeichi Nishi, an equestrian gold medal winner at the 1932 Olympic Games in Los Angeles. Kuribayashi welcomes Nishi and his horse, and the men reminisce about their days in the cavalry. Later over dinner, Nishi informs Kuribayashi that with the thorough defeat of the Japanese fleet at the Mariana Islands, it is evident that Iwo Jima will receive no naval or air support. When Kuribayashi announces his intention to build a series of caves and tunnels from which to mount their defense, Ohsugi and the other officers consider the action defeatist and doomed to fail. Despite their weakened physical state from limited rations and the constant nuisance of heat and bugs, most of the garrison commences the difficult work of digging into the island. Continually sickened by dysentery, Saigo’s friend Kashiwara dies. Later, Saigo and Nozaki suspect that replacement Pvt. Shimizu, from the prestigious police military academy, is a spy placed among the regular soldiers to report unpatriotic talk. When the men continue to receive meager rations, a hungry Saigo confides in Nozaki that he and Hanako ran a bakery until the military’s relentless requisitioning finally drove them out of business. Saigo also recalls the day he was called into service and his promise to his then unborn daughter to return home. Despite the navy’s success in forcing Kuribayashi to construct traditional gun casemates and pillboxes along the landing beaches, the officers remain deeply concerned with Kuribayashi’s methods. When Kuribayashi tells Ohsugi he is transferring him back to the mainland for health reasons, the admiral bluntly tells him that the cave defense is futile. Kuribayashi angrily responds that the longer the island can divert crucial American forces, the more time it will provide the homeland to gather its own defenses. Before departing, Ohsugi encourages Hayashi to continue resisting the general’s recklessness. As the work on the caves and tunnels continues, Nishi overhears hidebound Capt. Tanida instructing the soldiers, with the eager Shimizu reciting the military belief that Americans are cowardly and weak. Soon after, the American pre-assault begins with a heavy bombing campaign and Nishi is distraught when his horse, is fatally wounded in the first raid. The arrival of the American navy brings additional bombardments, increasing the tension throughout the caves. With the invasion imminent, Kuribayashi addresses his men over speakers, exhorting them to fight for their country and to kill ten Americans before they die. The enemy landing begins and despite heavy casualties from the unexpected, hidden defenses, the Americans gradually make their way toward Suribachi and the western side of the island. After days of desperate fighting, Tanida orders Saigo to request reinforcement from Adachi, but Saigo finds the commander despondent and prepared to order mass suicide despite Kuribayashi’s strict orders forbidding it. Returning to his unit’s cave with Adachi's note to Tanida, to Saigo’s horror, the men, including the frightened, weeping Nozaki, follow Adachi’s orders and begin killing themselves with grenades. After Tanida shoots himself, Saigo convinces the overwrought Shimizu to retreat with him. As the pair hasten through the maze of tunnels, they witness two soldiers set on fire with American flame-throwers. Moments later, when a Marine accidentally falls into a tunnel, the outraged soldiers brutally slay him. Saigo and Shimizu make their way to the cave commanded by Ito, who is furious that they have fled their position. His attempt to execute them for cowardice is halted by Kuribayashi, who announces that Mount Suribachi has fallen. Hayashi and Ito insist they must retake the mountain and although Kuribayashi refuses, that night Hayashi leads an attack outside. Ito follows, but when Hayashi and his men are decimated, he falls back and orders his men to join Nishi's unit. Ito then takes numerous anti-tank mines and, placing them around his neck, lies down among the dead men hoping he will be run over by an American tank which will detonate the mines. Angered over the waste of the mass suicides, Kuribayashi then receives a message from Tokyo that there will be no reinforcements, thus dooming the island to annihilation. At Nishi's cave, after Lt. Okubo wounds a Marine, Nishi surprises his men by ordering the soldier brought to the cave and treated with the last of their medicine. The young American is startled when Nishi speaks to him in English and relates his experience in Los Angeles during the Olympics. Meanwhile, Saigo contemplates surrender, but suspects that the stalwart Shimizu will prefer to commit suicide. Shimizu confides that he spent less than a week with the prestigious police military academy and was discharged and shipped to Iwo Jima after he proved incapable of killing a dog under orders. As the battle rages across the island, Kuribayashi recalls that upon leaving the United States in 1930 he was presented with a 1911 Colt 45, which he still carries with pride, as a parting gift from the officers at Fort Bliss, Texas. When the young American Marine in Nishi’s cave dies, the commander treats the body deferentially and upon finding a letter from the boy’s mother, reads it aloud to his men who are unexpectedly touched. The next morning when Ito finds himself still alive among the dead, he hurls the mines away and stumbles off. During another assault, Nishi is struck in the face and blinded and, learning there is no ammunition remaining, passes command to Okubo, ordering him to get as many men to the northern end of the island as possible. As Okubo leads the survivors, including Saigo and Shimizu, outside, they hear Nishi shoot himself in the cave. Disturbed, Shimizu admits to Saigo that he realizes he knows nothing about Americans and the exchange in the cave between Nishi and the Marine contradicts much of what he has been told about them. Not wanting to die in vain, Shimizu decides to surrender. Later that evening Shimizu successfully makes his way to the Americans where he and another soldier are provided water. Angered at being left behind to guard the two Japanese soldiers, however, an impatient Marine shoots the two men. The next morning, when Okubo’s men come upon the bodies, Saigo weeps bitterly for Shimizu. Later, Okubo leads the men on a desperate dash through enemy crossfire across open ground, but only Saigo and a few others manage to reach Kuribayashi’s headquarters. With no fresh water and reduced to eating worms, Kuribayashi continues to encourage the survivors and praises Saigo for being a good soldier. Privately, Kuribayashi writes to his wife not to expect his return. Later, the general and remaining men are deeply moved by a radio broadcast by the children of Kuribayashi's hometown of Nagano singing a song for Iwo Jima praising the soldiers’ sacrifice and honor. Determined to lead a final assault, Kuribayashi orders Saigo to remain behind and burn military documents and all his private papers, which include the company letters that have never been mailed. Although he burns the military documents, the private buries all of the letters in a bag. When Kuribayashi is wounded in the futile assault, Fujita drags him away to safety but the general insists on dying honorably. Fujita prepares to ceremoniously kill his general, but is shot before he succeeds. Later, Saigo finds the weakened Kuribayashi, who requests that the private bury him and, using the American Colt 45, Kuribayashi then takes his own life. Exhausted and demoralized, Saigo is picked up by Marines but, outraged when one soldier takes the Colt 45 as a souvenir, is knocked out before being taken to a medical unit. Years later, the research excavation team on Iwo Jima finds the bag of Kuribayashi’s papers and the letters from the soldiers to their families.