On Guadalcanal Island in 1942, a Marine patrol led by Lt. Joel Brady is fired upon by Japanese soldiers burrowed in a series of caves. When Brady decides to attack the caves, Sgt. Tom Sloan feels that the attack will result in senseless deaths and accuses Brady of “glory seeking.” After warning Sloan that he is bordering on insubordination, Brady leads a charge against the caves but is caught off guard when enemy reinforcements move in and kill all of his men except for Sloan and a Marine named Johnny Campbell. After surveying the dead, the traumatized Johnny angrily blames Brady and runs away. When Johnny ignores Brady’s order to stop, the lieutenant shoots him in the back, then warns the horrified Sloan to keep silent about the killing. Upon returning to camp, Brady reviews the attack with his superiors. When Maj. Westerly from battalion intelligence begins to question Sloan, Brady interrupts and recommends that Sloan be promoted, hoping to buy his silence. After Sloan is promoted to lieutenant and assigned to serve under Gen. Nathan Keller at a base in New Zealand, he vows to use his new rank to expose Brady’s treachery. Sloan, who had removed a bundle of undelivered letters addressed to Johnny’s wife Ruth from the Marine’s dead body, learns that Johnny’s widow lives near the base and decides to deliver the letters. Ruth, her younger sister Paula Nelson and father Casey Nelson welcome Sloan to their prosperous ranch. A grateful Ruth invites Sloan for dinner and asks him if Johnny died in vain. Later, Sloan is shocked to find Brady seated in the family’s living room. After Brady, who is dating Paula, explains that he met Paula several weeks earlier when he came to pay his respects to the family, Sloan accuses him of “riding Johnny’s corpse.” As the men talk, Ruth, who has lost both her husband and brother in war, warns Paula about becoming involved with a soldier. Some time later, at a briefing session at the base, the men are assigned to attack an enemy stronghold on the Tarawa atoll. When Sloan informs Ruth that he will be leaving on a mission, she becomes upset, but later steels herself and tells Sloan that she is only interested in a physical relationship with no hope of commitment or love. On the night that Sloan is assigned to a conduct a reconnaissance mission to obtain intelligence necessary for planning the raid on Tarawa, Ruth, dressed in a negligee, meets him in a hotel room to say goodbye. When Sloan chastises her for arranging a “quick shack up,” she breaks into tears and confesses that she is afraid of commitment. Taking her in his arms, Sloan proposes. The next day, as the troops board the ship for their mission, Brady, the officer in charge, informs Sloan that he and Paula were married the previous night. When Sloan accuses Brady of marrying Paula for her family’s money, Brady counters that Sloan profited from Johnny’s death because it freed Ruth. After landing at their objective, Sloan and staff photographer Sgt. Anderson infiltrate an enemy pillbox, a concrete emplacement housing machine guns, to take pictures for their attack on Tarawa, during which they hope to disable the enemy’s pillboxes so that the Marines can launch their offensive from the beach. When enemy reinforcements approach, Brady orders his troops to retreat, thus leaving Anderson and Sloan unprotected. Upon returning to the ship, Brady enters his cabin and is confronted by Sloan, who made it back safely in a second boat. As Sloan accuses Brady of trying to kill him, orders come for Sloan to report to Pearl Harbor for a briefing about the enemy pillboxes. There Sloan informs Nelson that he intends to bring charges against Brady for the murder of Johnny. Nelson, peeved that one of his officers will be subjected to a board of inquiry, informs Sloan that the charges will have to wait until he returns from Tarawa. Nelson and Sloan then join the others for the raid on Tarawa, and as Brady leads the charge on the pillboxes, Sloan stays on board with Nelson to plot strategy. After Brady’s company is pinned down by enemy fire, Nelson orders Sloan to go ashore and give Brady cover. Sloan leads his men in a charge against the enemy, after which Brady’s company moves up the beach to destroy the pillboxes. When Sloan loses contact with Brady, he dodges enemy fire to locate him. Wounded, Sloan finds Brady, the sole survivor of his company, hiding in one of the pillboxes, paralyzed with fear. Still unforgiving, Sloan informs Brady that he intends to file charges against him for Johnny’s death. When an order comes over the radio to attack the remaining pillbox, Brady surprises Sloan by charging out of the shelter and hurling a bomb into the target, thus wiping out the pillbox and freeing the Marines to launch their offensive. Brady is killed in the attack, and when Sloan returns to camp, Nelson asks his opinion about nominating Brady for a posthumous Medal of Honor. Sloan then replies that he is no longer sure about Brady's motives.