The film begins with a couple of text blocks explaining that in 2021 aliens invaded the planet. After crashlanding all over the earth, the next day begin the attacks by the aliens, which are called Heavies. After unifying the planets resources, men begins to fight back and the Heavies are driven off, leaving thousands of Heavies stranded on earth. After most of them have been disposed off, outposts are build to keep the rest of the world save from the remaining Heavies. Most of the people on earth move on and forget about the war, while the outposts are constantly undermanned and understaffed.
10 Years after the initial attack: Three new recruits and their documentary film crew are dropped of in the vicinity of Outpost 37 where they are ordered to serve. They get a short ride from a crew from another outpost, but need to leave the vehicle as enemy fire is detected in the area. They make it to outpost 37 to find it deserted and not respondent to hails. They enter and are pulled into a prank which the crew of outpost 37 plays on all their newcomers.
Later the camera crew is summoned by Captain Spears, who gives them each a gun, telling them that everyone on the base is a combatant. Shortly thereafter the base comes under attack by the local population. The outpost is build within their sovereignty and after 10 years of putting up with it, they are beginning to lose their patience with the soldiers. After calling in air support the locals are driven back, but not before wounding soldier Hollis, who needs to be transferred to the nearest other outpost, Mako, to have a bullet from his shoulder removed.
After returning, Captain Spears informs them that Hollis won't be returning, but will be recovering back in the civilized world. He also informs the men that he put in a request for reinforcements, but this request is denied.
The crew of the outpost teases Frankie, one of the new recruits, about the picture of his mother, which they took and put onto a poster of a topless model.
Later, a new soldier arrives on foot. He is questioned about his coming to outpost 37 and says he volunteered for the assignment, as no reinforcements will be send from command. Saleem, their local translator and friend, comes to visit and tells them they need to follow him to a man who says the soldiers killed his goats during the last attack.
An attachment of soldiers goes to meet the man, who says the soldiers owe hem twelve goats. Upon examining the goats, the men come to the conclusion they did not kill the goats and suspect a Heavy is in the area. With tensions high a young boy suddenly begins to move toward them. After being stopped by another man, he suddenly explodes and a Heavy appears. Taking heavy fire they retreat, only to find that soldier North has gone missing. After finding his helmet and examining the camera footage from his helmet cam, they see he's taken hostage by a Heavy. The crew wants to go find him, but are thwarted by high ranking officers who came to investigate the case. They are not allowed to leave the base.
At night, when looking at the camera footage of the area around base, they see Saleem being attacked by a Heavy. They disobey orders and leave to rescue him, as soldier Brick fires a drone, to stop the Heavy. The soldier save Saleem and take the heavily wounded, but still alive Heavy back to base. When Captain Sears finds out, he shoots the Heavy and tells them to follow their orders. Saleem then volunteers to look for North and goes out in his own.
When he comes back, he's checked at the gate for bombs, since the boy they saw earlier exploded. He checks out, but Brick, who is in the camera room, sees that Saleem has a very low body temperature. He tries to warn Frankie, but it's too late and Saleem shoots Frankie in the head. Captain Spears bursts out of his office and kills Saleem. After examining the body of Saleem, the doc finds a small incision in the back of Saleems neck. He doesn't know what to make of it.
After the funeral of Frankie, Captain Spears orders the men to gear up as they go out to find North. They find him, he is still alive but unconscious, and they bring him back to base. Though he's been shot three times, his vitals are stable and strong. He however doesn't regain consciousness. In the camera room, the men see North's eyelids moving and discover he's using tap code to give them a location. After informing Captain Spears he then asks North, who is still unconscious, what is at that position. Using his eyelid method, North spells out Steel Rain. He then wakes up suddenly and begins to attack Captain Spears, all the while screaming, kill me, kill me! Spears shoots him and North thanks him when he dies. The doc cuts open the scar in Norths neck, only to reveal an alien device which they think is used to control North with.
Spears wants to explore the location given to him by North, but is forbidden so by General Dane. A short while later a combined group of Heavies and locals begins to attack the outpost. After contacting command they are told the attacks are happening all across the outpost line and it will be at least an hour before they can get air support. Gearing up, the men blow cover fire as Andros rigs the base to explode. They leave at the last second and blow up the base, buying the other outpost some time. Brick is killed by a blast from a Heavy while in the truck.
Spears refuses to go to the Mako outpost and asks the men if they are willing to follow him to the coordinates North has given him. They all agree and make their way to the coordinates where they encounter a large alien structure, protected heavily by Heavies and locals alike. After making it into the facilities they find multiple rooms which house the missing soldiers and locals. They all have the same scar in their neck. After more resistance in the structure, they come across a power station, which is gearing up. Spears orders Andros to rig the station to blow as he and the other men give cover fire. After a heavy loss of casualties, in which one of the camera men is fatally wounded, most men run for the exit. The wounded camera man takes the detonation device and is seen exploding the charge as a Heavy comes upon him.
With the power station, the whole building blows and the soldiers, who are by now in their truck, see the building collapse. Moments later something big falls from the sky. After stopping, Omo investigates. It turns out the building was designed to disrupt the network of defense satellites who have been put around the earth after the first attack. In the moments the building was active, until the time it collapsed, hundreds of pods could come to the barrier to resupply the Heavies who stayed behind after the initial attack. The collapse of the building causes to defensive satellites to regain their effectiveness, stopping more pods and possibly new Heavies, from entering the atmosphere.
As the film comes to a close the camera man asks what Steel Rain means. The soldier answers with: Second Wave....