In the immediate aftermath of the Japanese attack on Pearl Harbor, Marine Sgt. John Basilone prepares to ship out and confront the enemy somewhere in the Pacific, while budding journalist Robert Leckie enlists in the Marine Corps. Eugene Sledge, unable to enlist because of a heart murmur, says farewell to his best friend, Sidney Phillips, who is about to leave for boot camp. Exactly eight months after Pearl Harbor, the 1st Marine Division, including Leckie and Phillips, lands on Guadalcanal in order to secure its strategically vital airfield and prepare for the inevitable counterattack.

Basilone and the 7th Marines arrive on Guadalcanal to reinforce Leckie and the rest of the 1st Marine Division as they continue to defend the crucial airstrip. Basilone plays a key role in repelling a nighttime Japanese attack, but suffers a frightful personal loss. After four months of continuous action, the exhausted and disease-ridden members of the 1st Marine Division are evacuated off the island.

Physically and mentally debilitated after the four-month ordeal on Guadalcanal, Leckie, Basilone and thousands of their comrades land in Melbourne, where they are greeted by adoring crowds and viewed as the saviors of Australia. While his buddies carouse, Leckie becomes deeply attached to an Australian woman and her first-generation Greek family. Meanwhile, Basilone is awarded the Medal of Honor and is asked to return home to help sell U.S. war bonds.

The Marines, including the newly christened “Sledgehammer,” continue the battle of Peleliu against an enemy determined to fight to the last man. Devastated by the loss of a revered leader, and witnessing unimaginable barbarity on both sides, Sledge veers to the very edge of moral collapse. Their objective finally secured, the Marines return to Pavuvu fundamentally changed by their experience on Peleliu.