The Archangel Michael (Paul Bettany) falls to Earth in Los Angeles and cuts off his wings. He steals a police car after a policeman is killed by another officer who is possessed. He travels towards the Paradise Falls Diner, near the edge of the Mojave Desert. Meanwhile, Kyle (Tyrese Gibson), a single father driving to Los Angeles, stops at the diner. He meets the owner, Bob Hanson (Dennis Quaid); Bob's son, Jeep (Lucas Black); Percy (Charles S. Dutton), the short-order cook; Charlie (Adrianne Palicki), a pregnant waitress; Howard (Jon Tenney) and Sandra (Kate Walsh) Anderson, a married couple; and Audrey (Willa Holland), their rebellious teenage daughter. An old woman (Jeanette Miller) enters the diner and begins taunting the diner's patrons. When Howard confronts her, she rips his throat open, screeches that they are all going to die, and climbs up the ceiling. Kyle shoots her before she can kill Jeep. Percy, Kyle, Sandra, Audrey and Charlie try to get Howard to the hospital but they are forced to turn back after passing through a swarm of flies.
Michael arrives and arms the patrons as the entire sky turns black. Hundreds of cars approach, filled with possessed people who begin to attack the diner. Michael leads the patrons in the fight, but Howard is dragged away. Later, Michael explains that God has lost faith in mankind and has sent His angels to destroy the human race. He also reveals that Charlie's baby must stay alive, as it is destined to be the savior of mankind; Michael disobeyed God's order to kill Charlie's baby, as he still has faith in humanity. The next morning, Sandra discovers Howard crucified behind the restaurant and covered with huge boils. She tries to rescue him, but he violently explodes into acid. Percy dies shielding Sandra from the blast. Sandra is driven insane and must be restrained. Meanwhile, the remaining survivors hear a radio transmission that reveals there are other pockets of resistance. One such refuge is nearby, but Michael advises them not to go, since they would be too vulnerable on the move.
That night, a second wave of possessed attack. Kyle is lured into a trap and killed and Charlie goes into labor. Audrey and Michael help her deliver the baby as trumpets sound, signaling the approach of the Archangel Gabriel (Kevin Durand). In a panic, Sandra breaks her restraints and tries to give the baby to the possessed, but Michael kills her. Moments later, Gabriel enters the diner and seriously injures Bob. Michael urges the group to escape and tells Jeep to "find the prophets, learn to read the instructions". The hordes of possessed humans are unable to approach Charlie's baby; Jeep, Audrey, Charlie, and the baby go to Michael's cruiser. Gabriel and Michael fight to a standstill before Gabriel stabs Michael through the chest with his morning star. Michael dies and his body disappears. Dying, Bob uses a lighter engraved "hope" to ignite the diner's gas main and blow up the diner, incinerating himself and the remaining possessed.
Jeep's body is covered in the same mysterious drawings seen on Michael's body; Jeep concludes that the tattoos are his instructions. Gabriel appears and flies onto the fleeing car. Audrey jumps on him and yells at Jeep to slam on the brakes, sending her and Gabriel through the front windshield as the car crashes and buying Jeep and Charlie time to get away but Audrey is killed in the process. Eventually Gabriel corners them in the nearby mountains and is about to kill them when Michael descends from Heaven, an Archangel again. Michael tells Gabriel that Gabriel gave God what he asked for but Michael did what God needed, giving humanity another chance; Michael says this was God's plan to test his angels and that Gabriel failed Him. Ashamed, Gabriel leaves. Michael explains to Jeep that he is the child's true protector and that they will see Michael again, before flying away. Charlie and Jeep reach the top of the mountain and see a small town in the valley below. Sometime later, Charlie, Jeep, and the baby drive away with a station wagon full of weapons.
Principal photography took place in New Mexico in the spring of 2008.
Legion was released on January 22, 2010 in 2,476 theaters and took in $6,686,233—$2,700 per theater its opening day. On its opening weekend it grossed $17,501,625—$7,069 per theater and placed second behind Avatar. It placed No. 6 on its second weekend, and grossed an estimated $6,800,000—$2,746 per theater, a 61.1% drop from the previous weekend. The film has come to gross $67,918,658 worldwide.
The film received negative reviews from critics. Review aggregator Rotten Tomatoes gives the film a score of 19% based on reviews from 101 critics, with an average rating average of 3.8 out of 10. The site's general consensus is: "Despite a solid cast and intermittent thrills, Legion suffers from a curiously languid pace, confused plot, and an excess of dialogue." Metacritic, which assigns a weighted average score out of 0–100 reviews from film critics, has a rating score of 32% based on 14 reviews.
Paul Nicholasi of Dread Central gave the film a one and a half out of five stars, saying, "The finished product is shockingly bad. If countless angles of people firing guns with spent shells clinking to the ground is all your heart yearns for, then Legion may be your ideal Saturday night. Hoping for anything more is an exercise in futility. Spare yourself the agony." Brad Miska of Bloody Disgusting gave it 1 out of 5 stars, calling it "a prude film with some potential. It's boring, slow paced and it takes itself way too seriously." Variety film critic Joe Leydon gave the film a mixed analysis. Leydon claimed "Even when the blood-and-thunder hokiness of the over-the-top plot tilts perilously close to absurdity, the admirably straight-faced performances by well-cast lead players provide just enough counterbalance to sustain curiosity and sympathy." Frank Scheck of The Hollywood Reporter also gave the film a mixed review stating, "the goings-on in Legion are seriously silly (not to mention more than a little derivative of endless movies, especially the Terminator series), but director Scott Stewart has provided enough stylish finesse to make the proceedings a real hoot."
Legion was released on DVD and Blu-ray on May 11, 2010.
In 2014, Syfy began airing the television series Dominion, a sequel set 25 years after the end of the film. Scott Stewart, the writer/director of Legion, served as executive producer. Stewart also directed Dominion's pilot episode, which was written by Vaun Wilmott and aired on June 19, 2014.