Former U.S. soldier John Rambo still lives in Thailand and now resides in a village near the Burmese border. He makes a living capturing snakes and selling them in a nearby village. He also transports roamers and fishes in his boat. He knows the river better than anyone. A missionary, Michael Burnett, asks Rambo to take him and his associates up the Salween River to Burma on a humanitarian mission to the help the Karen people. Rambo refuses but is convinced by Sarah Miller to take them.
The boat is stopped by pirates who demand Sarah in exchange for passage. After negotiation fails, Rambo kills them all. Although his actions save the missionaries, it greatly disturbs them. Upon arrival, Michael says that they will travel by road and will not need Rambo's help for the return trip.
The mission goes well until the Burmese army, led by the Major Tint, brutally attacks the village, killing most of the villagers and two missionaries, and kidnapping the rest.
When the missionaries fail to come back ten days after their planned arrival date, their pastor comes to Rambo to ask for his help in guiding hired mercenaries to the village where the missionaries were last seen.
Troubled by Sarah's potential fate, Rambo decides to accompany the soldiers. After seeing the destroyed village filled with mutilated humans and animals, Rambo encourages the platoon to move on. Hijacking a truck, they create a plan to save the hostages at the military camp, doing so within fifteen minutes to avoid alerting the army. Rambo helps Sarah and the others to escape. As soon as they leave the Myanmar Army unit finds their hostages missing and organizes a massive manhunt.
Everyone except for Rambo, Sarah, and "School Boy", the platoon's sniper, is captured. Just as the group is to be executed, Rambo hijacks a truck-mounted .50-caliber machine gun and engages the Burmese army.
A group of Karen rebles joins the fight to help Rambo and the mercenaries defeat the boats and infantry of the Burmese army. Seeing that the battle is lost Major Tint decides to flee, only to run into Rambo's machete, which Rambo then uses to disembowel the Major, killing him.