The film opens two years prior in Afghanistan, where a Staff Sergeant (Alan Ritchson) tries to rescue his brother (Jai Courtney) following a Taliban ambush. Despite carrying him miles toward safety, his knee gives out 1.8 kilometers short of the perimeter, and his brother dies from blood loss.
In the present, 81 enters RASP. Commander Sheridan (Dennis Quaid) and Sergeant Torres (Esai Morales) suspect 81 is mentally unfit due to severe PTSD. Sheridan grants him one final chance by assigning him squad leader for the "Death March" training exercise.
During the exercise, pieces of an extra-solar asteroid land nearby. The squad encounters a fallen pod, mistaking it for their target. The pod deploys a walking extraterrestrial combat machine armed with heat cannons and tracking systems. With only blank rounds, the candidates are hunted across forests and rivers, suffering heavy casualties.
Observing the machine during chase encounters, 81 notices that whenever its weapons fire continuously, it opens thermal exhaust vents to release core steam. Utilizing mud for thermal camouflage and improvised combative traps, 81 forces the mech to overheat, exposing its core and destroying it.
81 picks up the badly wounded Candidate 7 (Stephan James) and carries him across the RASP finish line, achieving emotional redemption. Upon returning to base, Sheridan reveals radar tracking data showing thousands of identical alien pods landing across the globe. The machine 81 defeated was merely an advance scout, setting up a global alien invasion.