The story opens with a team sent to a house in the countryside at midnight. They find a truck with one man dead and a trail of blood leading back to the house. Inside they find several targets (none of which are seen) and receive the kill-order. A series of gunshots rings out, accompanied by a series of screams. Moments later a young woman named Dana stumbles out of the house with blood on her hands, and gazes up at the sky in disbelief.
Two days earlier, Dana and her friends engage in a night of heavy drinking, fighting, and carousing at a club: Michael a lieutenant in the SAS, his best friend and warbuddy Robin, and Vincent. Robin proposes to Dana and she immediately accepts, while Michael hooks up with a beautiful American girl named Carrie. Vincent, however, acts very indecent towards a clubber and gets thrown out, along with Michael when he defends him. A fight ensues between Michael and the bouncers, with Robin and Dana joining in. Eventually the group heads back to Robin's house where Michael and Carrie have sex while Dana and Robin celebrate their engagement in a similar manner. Hung-over, they drunkenly stumble out of Robin's house, and discover that nobody on their street has mobile phone service or power. They are confronted by an apparently deranged tramp who insists that they are in danger from people with a purple mark. When a city-sized spaceship hovers over Derby, the city panics and society begins to break down. Michael, attempts to lead his friends to safety. They make their way to a store where their friend Pete works, but it is closed. The group rescues an immigrant from angry thugs, and Pete allows the group in through the side entrance. However, a riot breaks out when the crowd sees them gathering supplies, and looters attempt to steal their groceries. Michael frightens off the looters with a pistol, and the friends head back to Robin's house.
Michael and Carrie go out to get fuel and ammunition for Michael's handgun, and they run into John, a gas station attendant who believes that the aliens will attack. Carrie suggests that the aliens are explorers, but John says that there will be no lasting peace, because even if the aliens don't attack first the human governments will panic and attack the aliens inciting them to respond in kind. Back in the car, Carrie tells Michael that she is in England to learn about its culture and people, and they are involved in a car accident. Carrie frantically tries to save a survivor, but Michael administers a mercy killing when it becomes clear that will not make it. As Michael tries to drag Carrie away, she insists that there's a girl (seen earlier at the Market Square) still alive in the car. They rescue the girl, and the car explodes behind them. When they attempt to get supplies to bandage the injured girl, a policeman stops them, having locked up the place, but agrees to let them in when the girl identifies him as a "man with a purple mark". Once inside however, the policeman suddenly attempts to kill the girl. Michael engages in a long battle with him, only for the latter to overwhelm with superior fighting skills. Carrie saves Michael from being strangled by stabbing the policeman with a shard of glass, causing him to choke on his blood and suffocate. Carrie is left disgusted at Michael, and viewing him as cold-blooded killer, but Michael counters that it was "him or us".
Aliens target Robin's house, and he, Dana, and Vincent barely evade alien patrol ships. Robin and Vincent go out to steal a car, and Dana is left alone in the house, where she is apparently stalked by a spotter ship. She is saved by soldiers Kenny and Sam who shoot the ship down with a rocket launcher. The group reunites and heads off to George's house when the deranged tramp appears and confronts them with a pistol and says that they are protecting the Devil. Michael tries to reason with him, but the soldiers shoot the tramp dead as the tramp accidentally kills Robin. The group drives to George's house, where they become convinced that alien infiltrators are hiding among humans, identifiable via a purple mark. The group learns from the girl that the policeman they encountered earlier was one of them and had tried to kill the girl because she knew too much. George reveals that he has monitoring the situation with a special transmitter, supposedly alien in origin, and states that no one would know if it was happening. He concludes that they should only trust people that they know. The group turns on Carrie, for whom none of them can vouch and because the tramp had been pointing his gun at her. Michael convinces the others to allow him to privately inspect Carrie for a purple mark. As they enter the room, Michael confesses that he was dishonourably discharged, something he had neglected to tell anyone, even his friends. He states that he doesn't want to believe she is an alien but that he needs her to prove it, when Carrie abruptly shoots him dead with his own gun. Carrie fights off Sam, Kenny, and George who arrive to stop her, displaying the same combat skills that Michael had used earlier and survives a point blank shot from George's shotgun. She then takes Dana hostage and escapes outside George's house. Kenny follows her into a barn and shoots her just as she is teleported aboard a ship. George attempts to mollify the aliens by offering alien technology that he owns, but they disintegrate him.
Kenny and Sam attack the UFO with automatic rifles, but their weapons seem to have no effect. As they wait to be disintegrated, another UFO attacks and destroys that one. The sky fills with two different kinds of UFOs, which attack each other. Sam is killed in the crossfire, and the others retreat back to George's house. On his television, they see a newsreader in the form of Carrie announce that humanity has won the war and people should return to their homes. Vincent attempts to rape Dana, and Kenny savagely beats him and threatens to kill him. Before he can, an alien infiltration team led by a duplicate of the policeman breaks in and reports that there is a young girl there who can identify them. A series of flashbacks reveals there were several other copies of him who have followed the group and were presumably used as infiltrators. In the last scene, the infiltration team receives a go-ahead to kill everyone in the house, and their screams are heard over the radio. In the depths of outer-space, the battle between the two aliens rages on, as the mothership begins to descend.
Under the title U.F.O., the film premiered at the Prince Charles Theatre, Leicester Square, London on 13 December 2012, and went into general release on 24 December. In June 2013, the film was re-titled Alien Uprising and re-released in cinemas, as well as on video-on-demand services. It was released on home video 17 December 2013.
Neil Smith of Total Film rated the film 2/5 stars and wrote, "Alas, no amount of fiscal ingenuity can excuse the wooden acting and crummy dialogue in what is a feeble offering." Paul Mount of Starburst rated the film 3/10 and wrote, "U.F.O. is a misfire which has neither the coherent script nor the budget to even begin to make it work."