In Afghanistan, Taliban leader Ahmad Shah is responsible for killing over twenty United States Marines, as well as villagers and refugees who were aiding American forces. In response to these killings, the United States Navy SEALs are assigned to perform a counter-insurgent mission to capture Shah. As part of the mission, a four-man SEAL reconnaissance and surveillance team is tasked to track Shah's whereabouts. The four SEAL teammates are team leader Michael P. "Murph" Murphy; hospital corpsman and sniper Marcus Luttrell; sonar technician Matthew "Axe" Axelson; and communications specialist Danny Dietz.
The team is inserted into the Hindu Kush region of Afghanistan, where they make their trek through the mountains. Upon arriving at their designated location, the SEAL teammates are discovered by an elderly shepherd and two teenage goat herders. After much debate, Luttrell convinces the others that they will receive backlash if they kill the three herders. The team releases the herders, and as they attempt to abort the mission, they are ambushed by Taliban forces. They kill several approaching Taliban members, but find themselves too heavily outnumbered. All four men take a number of serious injuries during the firefight, and in an attempt to flee from the insurgents, they jump off the edge of a cliff.
Despite their injuries, the SEAL team runs through the woods. Dietz begins to lose consciousness and shout questions to Luttrell, unknowingly relaying the SEAL team's position to the Taliban. Murphy and Axelson jump off another cliff to flee from the Taliban fighters. Luttrell tries to carry Dietz down the mountain, but Dietz is shot in the shoulder; the impact forces Luttrell to lose his grip and fall forward off the cliff. A dying Dietz remains at the top of the cliff, in the custody of Shah and the Taliban insurgents who surround him. Murphy plots to climb back up the mountain in order to receive a phone signal to make an emergency call, via a satellite phone. Axelson and Luttrell shoot at the Taliban fighters, while covering Murphy. When he finally reaches higher ground, Murphy is able to alert the Navy SEAL base of his team's location, but is shot dead by several Taliban fighters.
In response to Murphy's distress call, a quick reaction force team attempts to extract the remaining members of the reconnaissance and surveillance team. During an attempt to insert SEAL teammates who were riding in one of two CH-47 Chinook helicopters, the Taliban insurgents shoot down one of the helicopters, killing eight Navy SEALs and eight Special Operations aviators who were on board; the second helicopter is able to escape. After witnessing the attack, Luttrell and a badly injured Axelson are left behind. Axelson attempts to find cover, but is killed when he leaves his hiding spot to kill several approaching insurgents. When Luttrell is discovered by the Taliban, one of the insurgents fires a rocket-propelled grenade, and its impact causes him to land at the bottom of a rock crevice where he is able to hide from the Taliban fighters.
Luttrell awakens the next day and runs to a nearby village, where he is discovered by local Pashtun villagers. One of the villagers, Mohammad Gulab, takes Luttrell into his home and sends a mountain man to the nearest American air base to alert military forces of Luttrell's location. The Taliban fighters arrive at the village to capture and kill Luttrell, but Gulab and the villagers intervene, threatening to kill the fighters if they harm Luttrell. The fighters leave, but later return to punish the villagers for protecting Luttrell. Luttrell and Gulab are able to fend off several fighters during the ensuing attack. The remaining Taliban fighters are chased away by American forces, who arrive via helicopters and bring Luttrell to safety.
Photos of the real-life Marcus Luttrell, Mohammad Gulab and the fallen soldiers who died during the mission are shown during a four-minute montage, and an epilogue reveals that the Pashtun villagers agreed to help Luttrell as part of a traditional code of honor known as the Pashtunwali.