Aug 12, 2011. Los Angles is under attack by an unknown enemy. Marines in full combat gear scramble to their helos. Staff Sgt Nantz (Aaron Eckhart) and his platoon sit nervously as they fly through gunfire along the city.
24 hours earlier:
Nantz and his buddies exercise on the sunny beach. Younger men leave him in the dust, panting and sweating. He drives to Camp Pendleton and listens to a radio report about a meteor shower. In the office another Sgt can't believe Nantz has filed his retirement papers.
The platoon is in a flower shop, one of the men is getting married. They joke around. In the barracks rec room others joke around and relax. Lance Cpl. Kerns (Jim Parrack) is meeting the Psych Doctor and claims he's fine. Cpl. Lockett (Cory Hardict) talks to his dead brother at a cemetery. That night, the boys play golf and have beer. Corpsman Jibril (A. McCormack) wants to be a doctor.
The TV news says there are evacuations along the coastline, the meteors should hit a few miles offshore. The next day Nantz trains a platoon in a sandpit when they are ordered to mobilize. Nantz is needed. Lt. Martinez (Ramon Rodriguez) of 1st Platoon is right out of OCS and asks one of his men what they know about Nantz, who will be filling in as platoon sgt. The man hints at a shady past. The men gear up as reports of meteor showers around the world come in. In Iraq it turns out Nantz was a hero but got a couple of guys killed in action.
The meteors hit off Santa Monica. A briefing officer says they slow down before hitting and are mechanical in nature. It is threat con delta! The TV shows panic on the beaches as large figures walk toward the shore firing laser guns and guided rockets at the humans. Martinez gives the men the gung-ho speech and they board the helo. Bravado slowly turns to fear and Martinez writes a letter to his wife. Over Santa Monica beach, they look down at fiery wreckage and the helicopters fly though incoming and outgoing barrages as well as enemy ground fire. They land at Forward Operating Base, Santa Monica Airport.
An officer gives a sitrep and briefs them on their task, Santa Monica is going to be levelled to stop the infestation of the invading aliens at 19:30 hours. Lt. Martinez is to take his squad to a police station at Venice and Amorosa, to save some civilians. They have exactly three hours before bombing starts.
The troops move out as panicked people run the other way. They walk warily through smoke and destruction. Suddenly they are under attack from bolts of tracer-laser fire from all directions by unseen hostiles. One alien soldier is seen on a nearby rooftop of a house and shooting down at them, as the Marines scream and regroup. Martinez freezes up as the squad is boxed in. They manage to break thru a fence and escape and take cover in a house. Linehan is missing, Nantz goes back to get him with two others. Lineham is cornered in a laundry room. Linehan comes face to face with an alien and manages to wound it by shooting at it and throwing it into a swimming pool. The other three arrive and all four empty their mags into the creature. They grenade the pool for good measure.
The squad keeps moving toward the police station and some are injured. They have to take cover again when they come under fire again from bearly seen alien troops in which three more marines from another squad join them. Tech Sgt Santos (Michelle Rodriguez) is one of them. They proceed to the police station. After a quiet walk for about 15 more minutes, the Marines finally arrive at the police station only to see the place all shot up. They find five civilians, a couple and three kids. An evac helicopter arrives and some of the squad get in but are immediately blasted by enemy aircraft which now appear out of the skies. Martinez is panicky but Nantz settles him down. They take to the roof of the police building to plan a way back to the FOB (Forward Operating Base). Nantz introduces himself to the civilian Joe Rincon (Michael Pena) and his son Hector (Bryce Cass). The marines on the roof watch a squad of aliens setting up on a roof 300 meters to the north. On a TV screen, the CNN expert says Earth is being colonized for its resources. The aliens apparently want the water which they use as fuel for their spaceships and weapons.
The marines plan to escape using a bus, but they have less than an hour before the airstrike hits. As two go to hotwire the bus, the others find a wounded alien. Nantz wants to find out where their weak point is. The civilian Michele (Birdget Moynahan) is a veterinarian and helps as they rip into the body through its binary mechanical suit which has its assault weapon emplanted in it. Just then, the alien reinforcement soldiers attack the police station. Nantz uncovers a translucent organ and bayonets it, just to right of the heart, thus killing the captive alien at last.
Everyone piles into the bus and off they go. They need to travel six blocks in 35 minutes. They hear a funny noise and stop as a large squadron of enemy aircraft fly overhead heading east to downtown LA. They realize the aliens are homing in to their radio transmissions, Nantz grabs a radio and runs ahead, he sets it on to draw a flying craft close, then blows it up as it passes by a gas station. He investigates the wreckage and discovers it is a UAV (an unmanned aircraft). The marines cheer his "John Wayne" heroism.
Nantz and Santos quietly discuss the aliens must have a command and control center. They have 25 mins and must detour on to the elevated freeway. Getting close, the off ramp is blown up and the bus is attacked by alien soldiers on another overpass whom are shooting at an engaging battle tank. Two marines try to set up a rappel to get down to the ground as the rest provide cover fire. After brief success, a large alien walking tank comes towards them after detroying the M1 Abrams tank. The marines pull back and place bombs in damaged cars. Rincon grabs a gun and shoots an alien but is mortally wounded. Martinez is badly wounded and tells Nantz to save the rest. He gives the SSgt the letter to his wife. As the aliens approach, Martinez triggers the bombs in suicide and saves the squad. The surviving marines whisper that Nantz left the Lt. to die.
Once again they all take a rest in an abandoned building which is a local supermarket. Nantz tries to relieve the dying Rincon. Santos finds a landline. Turning on a TV set in the store, the CNN TV experts say the aliens are already drawing down the ocean levels. There are 20 large alien ships off each major coastal city. Santos and Nantz plan to take it out. There is one minute to the deadline. Countdown 3-2-1-0....nothing happens. At nightfall, they arrive at the Santa Monica airport FOB, it is deserted and in complete ruins after an alien airstrike. Hector cries by his father's body, Nantz comforts the boy. A marine calls out Nantz and his apparent disregard for others. Nantz proves each time one of his men died has deeply affected him, including the Corporal's brother back in Iraq. They find a LAV armoured vehicle and Humvee and head to the next evac site.
The LAV runs over a group of aliens as they travel and shoot at high speed. A Huey comes to rescue them. The pilot says they are abandoning Los Angeles. In the air, Nantz looks solemnly at LA in flames. They fly over an area of high RF interference, Nantz wants to go down and recon. He gives Martinez' letter and rappels down alone, shortly after the rest of the Marines follow him.
They sneak up and observe the aliens entrypoint to the underground Command and Control point. They move in through the sewer and get close to a huge mechanical device. Now they go up to the surface and plan their tactics. Kerns calls in the target co-ordinates and the Marines defend their position as they aim a laser. Just then, a huge alien craft begins to break out of the ground ascend and the marines try to keep the laser aimed at it. Santos takes out an alien defensive drone with her Stinger missle and finally a copperhead laser guided missle hits the alien c&c craft which explodes. The alien troops retreat.
Nantz and his squad are evac'd to the Mojave Desert. After informing forward command of their success, it is relayed to the armies of the rest of the world which will copy the technique to defeat the aliens. Nantz starts to reload his magazine, the others join him to go back into the fight and take back LA.