Directly following the attack on Pearl Harbor, Seargent John Basilone ships out to fight the enemy somewhere in the Pacific, and a young journalist called Robert Leckie enlists in the Marine Corps. Sidney Phillips ships off to boot camp after saying farewell to his friend Eugene Sledge, who cannot go with him due to a heart murmur. Eight months later, Phillips and Leckie, having completed boot camp are sent to secure a airfield on Guadalcanal, and to help defend it against counterattack.

Back-up lands on Guadalcanal in the form of the 7th marines including Basilone to help defend the airfield. Basilone helps defend against a Japanese night raid, suffering a loss in the process. The marines, after four months of fighting enemies and disease are evacuated off the island.

Leckie, Basilone and thousands of other soldiers land in Melbourne physically and mentally drained. On arrival they are greeted by crowds of people cheering and treating them as heroes. Basilone is awarded the Congressional Medal of Honour and is asked to return home. Meanwhile Leckie developes strong feelings for an Australian woman.

Having finally enlisted as a marine, Sledge is sent to Camp Elliot to train for battle. The 1st Marine Corps ships out to battle the Japanese who are holding an island off New Britain called Cape Gloucester. The marines fight bravely, but find the jungle to be the greatest enemy. Leckie having survived Camp Gloucester begins to show the effects of combat and is sent to a naval psychiatric hospital.

Basilone travels the country selling war bonds as his celebrity increases. Sledge has finished his training and is now part of the 5th marines, 1st marine division and is reunited with his friend Phillips. Leckie also rejoins the company. The marines land on the heavily defended island of Peleliu, where Sledge gets his first taste of combat against the Japanese.

The Marines battle against heat and unclean water and try to take Peleliu airfield. They are met with heavy opposition. Leckie sees many of his comradeshuged or injured.Leckie, injured himself is then evacuated off the island, contemplating the notion of retreating to fight another day.

The battle continues against the extremely determined Japanese. Terrible acts of barbarism are conducted by both sides and Sledge questions his own moral compass after the loss of a leader. The Marines eventually secure Peleliu with the help of their newly christened 'Sledgehammer' and return to Pavuvu, changed men forever.

Basilone persuades the Marines to allow him back into the ranks after becoming increasingly tired of selling war bonds. He enjoys a romance while stationed at Camp Pendleton with a female marine called Lena Riggi. The two marines know their time together will be short, as Basilone will soon ship out to Iwo Jima.

Sledge and the 1st Marine division battle in Okinawa for over a month when they are ordered to relieve another company to battle an extremely heavily defended Japanese position on the island. Civilians on the island cause many moral dilemmas and make things much more difficult for the marines.

The Japanese have finally surrendered. Leckie adjusts to a normal life back at his old job after healing from his wounds at the hospital. Sledge returns home and is reunited once againd with his good friend phillips. Basilone is visited by Lena and an emotional reunion follows.