Major Steve Lockhart, [Pat O'Brien], commander of a Marine Raider battalion, and Captain Dan Craig, [Robert Ryan], commander of the Paramarines, stand on Guadalcanal as a fierce Japanese assault pushes toward what history would call the Battle of Edson's Ridge. When Craig discovers the body of Lt. Tony 'Junior' Hewitt, [Russell Wade], tortured and executed by the Japanese, his grief erupts into a one-man, vendetta-fueled rampage through the jungle, armed with his Reising gun (notoriously prone to jams) and his M1911 pistol.
Major Lockhart harshly criticizes Craig for such conduct, arguing that vengeance has no place in combat. The only thing keeping Craig from being relieved of command is the timely success of a Japanese night attack that is beaten back during a brutal, exhausting night of fighting. The fighting-weary units then travel to Australia for rest and recreation, seeking respite from the toll of war.
In Australia, Craig is drawn to Ellen Foster, [Ruth Hussey], a capable Flight Officer with the Women's Auxiliary Australian Air Force who has two brothers serving in North Africa. The two fall in love and dream of marriage, but Craig sustains injuries in a Japanese air attack that complicate their plans. When Lockhart visits him in the hospital and learns of his intention to wed Ellen, he reluctantly concludes that Craig might be unstable and transfers him back to the United States, [Pat O'Brien], hoping to steady the situation.
Back in the United States, the recovered Craig and Lockhart arrive at Camp Elliott as bitter enemies, shocked by how rapidly the Marine Corps has expanded and how many men are being trained. The film includes candid glimpses of the training process at the base, giving a window into the scale of the Corps’ postwar buildup. Despite their dispute, Lockhart eventually defends Craig when others question his fitness for active service, and he urges that Craig be kept in the fight rather than sidelined.
Preparations for an invasion of a South Pacific island take shape while Craig and Ellen marry in a quiet ceremony in Australia, a union that both strengthens and complicates their lives. Lockhart’s outlook softens a bit on marriage after meeting Ellen, and the two men move toward reconciliation even as duty pulls them in different directions.
The climax centers on a map-identified Bougainville operation, where Craig's Paramarines leap behind Japanese lines to ease pressure on a beachhead where Lockhart's Raiders and other Marines have already landed. The mission underscores the ongoing push of the war effort, and back in Australia, Ellen listens with pride to radio reports announcing the exploits of her loved ones, affirming that love and duty can endure the most trying tests.
