Australia and New Zealand deployed their troops to Vietnam in support of the United States' effort against communism, consisting mainly of young conscripts and volunteers, with an average age of merely 20 years. Most of these soldiers enter the fray with little to no experience in combat situations.
The film begins dramatically with a mortar strike targeting the 1st Australian Task Force (1 ATF) base at Nui Dat, carried out by the Viet Cong (VC) and the People's Army of Vietnam (PAVN). Amid the chaos, Major Harry Smith, played by Travis Fimmel, remains composed as he directs his unit, Delta Company, to seek cover, while other soldiers, unfazed, indulge in card games and beer.
The base is manned by a variety of personnel, including Captain Morrie Stanley (Aaron Glenane), Warrant Officer Class Two Jack Kirby (Alexander England), Bombardier Ray Ngatai (Uli Latukefu), Private Paul Large (Daniel Webber), and Private Noel Grimes (Nicholas Hamilton). New recruits Private Frank Topp (Jullian Cullen) and Private Colin Whiston (Sam Fraser) also join Delta Company.
Meanwhile, Second Lieutenant Gordon Sharp (Mojean Aria), tasked with locating the mortar's source, is distracted, opting to play cards with his men instead of following orders from Major Smith. Luckily, Sergeant Bob Buick (Luke Bracey) steps in, assisting Harry in tracking the mortar's origin. In charge of the artillery unit is Lieutenant Colonel Colin Townsend (Anthony Hayes).
The Australians are ultimately able to repel the initial attack through counter-battery fire, successfully neutralizing the enemy mortar crew. However, the casual attitude of many toward the war is evident; Harry reprimands Private Large for drinking during patrol duty right as they're under attack. Kirby, concerned about Harry's leadership, confronts him, stating that he runs his company more rigorously than any other.
Feeling unappreciated among conscripts, Harry seeks a transfer to a commando unit, but his request is denied by Brigadier Jackson, who orders him to adhere to Townsend’s commands. Following the attack, Major Smith volunteers Delta Company, 6th Battalion, Royal Australian Regiment (6 RAR) to scout a rubber plantation near Long Tân, but is turned down; instead, Bravo Company receives the task and finds no enemy presence.
The following day, Delta Company is directed to replace Bravo and misses an anticipated performance by Australian musicians Little Pattie (Emmy Dougall) and Col Joye. Tracking fresh enemy footprints, they soon engage in combat, with 11 Platoon, under Sharp's leadership, first making contact with a VC patrol. Heavy machine gun fire ensues, and though Harry dispatches 10 Platoon for reinforcement, they, too, find themselves in intense skirmishes with the enemy.
As hostilities escalate, 11 Platoon's situation worsens, forcing them to call for artillery fire, which is precariously close to their position. This risky maneuver effectively stalls the advancing VC but comes at a cost, with bleak news filtering back to Harry about the overwhelming enemy numbers.
Determining they face a battalion of 700 to 800 soldiers, Harry urgently requests reinforcements, including Armored Personnel Carriers (APCs) and air support. Although Jackson eventually agrees to send air support, the situation becomes precarious, as their reconnaissance efforts fail to locate necessary markers. Realizing the grave situation, Buick suggests they target their own men with artillery fire, a heart-wrenching choice that Harry must concede to.
As tension mounts, with the base under threat of VC encirclement, Harry orchestrates a withdrawal to avert further losses. Under now dire conditions and low on ammunition amid monsoon downpours, they seek helicopter resupply. Although initially hesitant, two brave RAAF pilots step in to deliver essential supplies under enemy fire.
Rearmed yet still outmatched, Harry devises a rescue operation to save 11 Platoon, and against all odds, they locate survivors. News of their finding breathes new life into the beleaguered base, inspiring the troops. Ultimately, a reluctant Jackson must send APCs to aid Delta, risking the loss of the base itself.
As the battle culminates, with Harry bravely fighting off VC forces, reinforcements finally arrive. Tragically, despite the victory, a beloved soldier, Largie, falls in the line of duty. In a poignant finale, the film pays tribute to the sacrifices made, revealing the harrowing truth of the Battle of Long Tan, where a mere 108 Australian and New Zealand soldiers bravely withstood the onslaught of over 2000 North Vietnamese soldiers, resulting in significant casualties on both sides.
