The main character, Jack Harper (Tom Cruise), tells viewers that Earth was nearly destroyed during a devastating war started by alien invaders. Although the humans eventually won, he says, the planet is in ruin because the moon was destroyed and large sections of the Earth were contaminated by nuclear fallout. Jack and his partner, Vika (Andrea Riseborough), are the only humans stationed at an Earth outpost charged with maintaining drones. These machines are programmed to engage enemy that remained on the planet after the war. Jack says his mission is crucial because these beings want to destroy power generators that extract energy from Earth and ship it to the planet Titan, where most humans now live.
Although Jack is diligent about his duties, the feeling that he is somehow connected to Earth plagues him. This is particularly true when he thinks about the impending conclusion of his mission, and his concerns come to the forefront when an ancient human vessel crashes to Earth with most of its crew still in suspended animation. Although a drone kills nearly everyone aboard, Jack rescues one woman (Olga Kurylenko), and his interaction with her takes him on a path he never imagined.