An alien apocalypse is happening, and its coming in waves. Everyone on Earth is dying rapidly, while the citizens try to take action, but fail. Cassie Sullivan and her little brother, Sammy Sullivan, fight to stay alive. Their parents die and Sammy gets separated from Sammy and gets transported to Camp Haven. Cassie gets wounded by an alien and gets taken in by that same alien, Evan Walker. Evan takes care of Cassie at his old house, while Sammy gets put in boot camp and meets Ben Parish, who Cassie had a crush on in high school. Cassie is determined to keep the promise that she made to Sammy about coming back for him. Together, Cassie and Evan, with his resources, infiltrate Camp Haven and defeat the secret aliens that look like humans, and run the camp. They rescue Sammy and Ben, due to Cassie's determination to come back to Sammy.