December 1941. In the aftermath of the Japanese attack on Pearl Harbor, Robert Leckie tells his neighbour Vera that he has enlisted in the Marines and impulsively suggests that he might write to her while he's overseas. Sargent John Basilone and his Marine brothers-in-arms J.P. Morgan and Manny Rodriguez listen as their commander Lt. Col. Chesty puller explains that they will be sent to the Pacific Theatre of Operations, where they will be fighting for "tiny specks of turf that we have never heard of."
Meanwhile, Alabama teenager Eugene Sledge is devastated when his physician father. Dr. Sledge confirms he still has a heart murmur that will prevent him from enlisting in the Marines like his best friend Sid Phillips.
Several months later, Phillips is assigned to the same company as Leckie. Along with their pals Runner, Chuckler, Hoosier, and Gibson, they are part of the invasion of a distant island called Guadalcanal. After an unopposed beach landing, the men move into the jungle and are stunned and angered when they encounter the mutilated bodies of fellow Marines. Later, Leckie and his men watch from a hilltop as another regiment takes the airfield. That was the objective of the invasion. The U.S. Navy suffers a major defeat and retreats to open water, leaving Leckie and the others to defend the airfield, abandoned and unsupplied. The Marines repulse a furious night-time Banzai attack, but it's only the beginning. The psychological and emotional toll exacted on the men by their first taste of combat is clear. The post-battle carnage that covers the beach prompts Leckie to reflect on the nature of war itself. He begins writing the first of many letters to Vera.
After a month of increasing deprivation on Guadalcanal, Leckie and his men are reinforced by Chesty Puller's Seventh Marine Regiment including John Basilone.