The film opens with a mortar attack on the 1st Australian Task Force (1 ATF) base at Nui Dat by the Viet Cong (VC) and People’s Army of Vietnam (PAVN). During the attack, Major Harry Smith is cool under fire, ordering his men to take cover and stand to, while other soldiers don’t take it seriously, playing cards or drinking beer. The base fends off the attack with counter-battery fire, killing the enemy mortar crew.

Following the attack, Major Smith volunteers his unit, Delta Company, 6th Battalion, Royal Australian Regiment (6 RAR) to investigate the rubber tree plantation at Long Tân 2 kilometres (1.2 mi) away. Instead, Bravo Company is ordered to locate the mortar firing points and the direction of the enemy withdrawal. Bravo finds no enemy forces.

The next day, Major Smith’s Delta Company is sent to relieve Bravo, and thus missing the concert of Australian musicians Little Pattie and Col Joye and the Joy Boys set for that afternoon. Finding fresh tracks leading away from the mortar firing sites, Delta moves to follow the enemy forces. 11 Platoon takes the lead, and makes contact with a small VC patrol. 11 Platoon penetrates further into the plantation, widening the gap with 10 Platoon, and the rest of the company.

11 Platoon quickly comes under heavy attack, and calls down fire from artillery units back at Nui Dat, danger close to its position to hold back the enemy force. 12 Platoon reinforces, but the attack is only getting stronger. 11 Platoon become isolated, in danger of being overrun.

Back at Nui Dat, Brigadier Oliver David Jackson is hesitant to commit reinforcements, and orders Delta to withdraw. Major Smith refuses to leave 11 Platoon behind, eventually forcing Jackson to send APCs to rescue Delta, leaving the base open to attack. Low on ammunition and under a monsoon, Major Smith pleads for a helicopter resupply. The senior RAAF officer at Nui Dat, Group Captain Peter Raw, does not want to risk aircraft with a hot LZ in the monsoon. However, two RAAF Iroquois pilots, Flight Lieutenant Francis Patrick (Frank) Riley and Flight Lieutenant Robert George (Bob) Grandin, volunteer to support D Company, flying in under heavy fire.

Now resupplied, but still outnumbered, Major Smith organizes his forces to successfully hold off the assault, before the relief force of M113 armoured personnel carriers and infantry from Nui Dat finally arrives, and force the PAVN soldiers to withdraw — but not before Largie is shot in the head while fighting, and dies on the spot. In a final aftermath scene, the soldiers are roll-called, and those that were KIA are missing. Major Smith bids one last farewell to departing body of Largie by helicopter.