Following the events of Alien vs. Predator, a Predator ship is leaving Earth carrying Alien facehuggers, and the body of Scar, the Predator that defeated the Alien Queen. A chestburster erupts from Scar’s body; owing to the Xenomorphs abilities to take characteristics, this chestburster has the traits of both species. It quickly matures into an adult Predalien, makes its way onto a scout ship that is then detached from the Mothership, and starts killing some of the Predators on the ship. A Predator’s weapon fire punctures the hull and the scout ship crashes in the forest outside of Gunnison, Colorado, injuring or killing most of the Predators on board.
With the Predators dead, the Predalien and several facehuggers escape, implanting embryos into a nearby father (Kurt Max Runte) and son (Liam James) and several homeless people living in the sewers. An injured Predator sends a distress signal before being killed by the Predalien. The distress signal reaches the Predator home world and a lone Predator, Wolf (Ian Whyte), responds, traveling to Earth to track and kill the Aliens. He begins to erase the evidence of the Aliens’ presence by destroying the crashed ship and using a blue liquid to dissolve the bodies of the facehuggers and their victims.
Meanwhile, ex-convict Dallas Howard (Steven Pasquale) has just returned to Gunnison after serving time in prison. He is greeted by Sheriff Eddie Morales (John Ortiz) and reunites with his younger brother Ricky (Johnny Lewis). Ricky has a romantic interest in his classmate Jesse (Kristen Hager) but is constantly harassed by her boyfriend Dale (David Paetkau) and two of his friends. Kelly O’Brien (Reiko Aylesworth) has also just returned to Gunnison after serving in the military, and reunites with her husband Tim (Sam Trammell) and daughter Molly (Ariel Gade).
Wolf fights a number of Aliens in the sewers, and as the battle reaches the surface several of them disperse into the town. Wolf pursues some to the power plant, where collateral damage from its weapons causes a citywide power outage. Ricky and Jesse meet at the high school swimming pool but are interrupted by Dale and his cohorts just as the power fails and an Alien enters the building, killing Dale’s friends. Another Alien invades the O’Brien home, killing Tim while Kelly escapes with Molly.