SAS lieutenant Michael Galloway, along with his friends Vincent and Robin, and Robin's girlfriend Dana, decide to have a night out at a club. During the evening, Michael forms a connection with a girl named Carrie, while Robin takes the opportunity to propose to Dana. Just as Robin witnesses a peculiar light in the sky, a brawl breaks out, prompting him to assist Michael. Subsequently, the group returns to Robin's house, where Michael and Carrie share an intimate moment, and Robin and Dana celebrate their upcoming nuptials.
Upon waking the next day, they quickly realize that communication systems like phones, televisions, and radios have ceased functioning. Michael and Carrie, slightly hungover, venture outside and are warned by a tramp about impending danger from individuals bearing a purple sign. Suddenly, an earthquake jolts them, leading them to discover a massive spaceship hovering over Derby. As society begins to unravel, they head to a local store to gather supplies, aided by Michael's friend Pete.
However, things take a dangerous turn when two looters wait for them outside, but Michael manages to scare them away by brandishing a pistol as they return to the safety of Robin's house. That night, Michael and Carrie set out to secure fuel and ammunition. They encounter a gas station attendant who ominously informs them that war is imminent; regardless of whether the aliens start it, panicking human governments will undoubtedly make the first move against them.
After a car accident, Michael is compelled to perform a mercy killing on a critically injured survivor. They find themselves rescuing an injured girl from the wreck, but their attempts to procure medical supplies are thwarted by a police officer. The officer, however, allows them to pass once the girl reveals he is "a man with a purple mark." In a shocking turn of events, the officer tries to kill the girl, but Michael and Carrie intervene, taking the officer down.
Back at Robin's home, the group witnesses alien crafts encroaching. Robin and Vincent take a risk to steal a car while Dana is stalked by a spotter ship, but she is saved by soldiers Kenny and Sam, who successfully bring the ship down using a rocket launcher. Once reunited, the group devises a plan to take refuge at Michael's uncle George's house. However, a tramp sneaks up behind Michael, seizing his pistol and menacingly declaring they are protecting the Devil. In the ensuing chaos, the tramp is shot by soldiers, yet tragedy strikes when Robin is shot and killed.
Upon their arrival at George's residence, he reveals that he has been using an alien transmitter to monitor the situation. He shares his insights on the aliens hiding among humans, identifiable by the purple mark, urging them to trust only those they know. Suspicions quickly turn toward Carrie, as none can vouch for her. In a moment of vulnerability, Michael confides in Carrie about his past dishonorable discharge and feelings of shame. However, the truth takes a dark twist when Carrie reveals her true intentions and shoots Michael.
As chaos unfolds, she manages to escape with Dana, taking her hostage. In a desperate attempt, Sam and Kenny pursue her to a barn where Carrie is mysteriously teleported away to a spaceship. George’s desperate offering of technology to the aliens results in his tragic demise when the ship disintegrates him. Their firearms make no impact against the UFO, leading to an unexpected battle between two UFO factions that fills the sky, with Sam tragically succumbing to the chaos.
The group ultimately retreats back to George’s house, where tension escalates further as Vincent, driven to madness by the recent events, attempts to assault Dana. However, Kenny intervenes, threatening Vincent to protect everyone. Suddenly, the power rekindles, and they are shocked to see a news report announcing that humanity has emerged victorious in the war, urging everyone to return to their homes.
The situation deteriorates as an alien infiltration team, led by a duplicate of the policeman, breaks into the house, revealing they have been tracking the group since the nightclub incident. The team receives orders to exterminate everyone present, and soon their screams fill the airwaves. Meanwhile, high above them in the cosmos, the battle between the two alien factions escalates as the mothership descends, foreshadowing further conflict.
