In a voiceover, “Tech 49” Jack Harper tells of the state of the Earth on March 14, 2077. Sixty years earlier, a race of alien scavengers destroyed the Moon and invaded Earth to seize its resources. Humanity won the war, but the nuclear weapons they used, and the loss of the Moon, have left most of the planet in ruin.
Harper and his housemate and communications partner Victoria “Vika” Olsen are among the few people left on Earth. Working as a team, they repair the network of combat drones that hunt the remaining “scavs” and guard the fusion generators (“hydrorigs”) that draw seawater to extract energy. Led by mission director Sally aboard the Tet, a giant tetrahedral space station that orbits Earth, they are about to complete their five-year mission and return to the Tet to travel to humanity’s new home on Saturn’s moon Titan.
Although their memories have been wiped to help them focus on their mission, Jack’s affection for Earth and the remnants of its history that he finds make him uneasy. He also has recurring dreams of a time before the war and visions of an unknown dark-haired woman.
While on duty, Jack discovers the scavengers are using the Empire State Building’s antenna to transmit coordinates within Grid 17 to outer space. After he dismantles the transmitter he tells Vika he’ll perform a perimeter check around the border where the “radiation zone” meets the safe area. Instead, he visits a secret cabin he has built in a hidden lush green valley and filled with mementos from Earth’s past.
Jack soon awakens at the cabin to see unknown objects parachuting from the sky. When he finds the Odyssey in Grid 17, he sees it holds five humans in stasis chambers, one of which is the woman from his dreams. Though a drone arrives and begins to kill the survivors, Jack protects the woman’s chamber. When he brings her to the tower, she awakens, recognizes Jack, and says her name is Julia. Against Jack’s wishes, Vika reports Julia’s presence to Sally, who requests Jack take Julia to the Tet.
Early the following morning, without Vika’s knowledge, Jack and Julia return to the crash site and recover Odyssey’s flight recorder. Soon afterward, the scavengers capture them and take them to the Raven Rock Mountain Complex, where they reveal themselves to be human. Their leader, Malcolm Beech, who was a young soldier during the war, asks Jack to reprogram a captured drone to carry nuclear fuel cells to the Tet to destroy it. When Jack refuses, Beech releases Jack and Julia to seek the truth in the supposed radiation zone. When they return to the remains of the Empire State Building, Julia reveals she is his wife, causing Jack to remember him proposing to her there.
When they return to the tower, Vika refuses to let them enter, having witnessed the couple’s exchange through the camera on Jack’s craft. Instead, she tells Sally she and Jack are “no longer an effective team”. Sally activates a drone that kills Vika before Julia destroys it. Though Jack and Julia escape in his craft and destroy two of the three drones that pursue them, a collision with the third forces them to eject over the radiation zone. After they land, they see another ship with a technician—a clone of Jack named “Tech 52″—who attempts to repair the drone. Jack incapacitates his clone, but Julia is shot during the fight. When Jack uses Tech 52’s craft to travel to his clone’s tower for medical supplies and finds a clone of Vika, he tricks her into believing he is “her” version of Jack.
At the base, Beech tells Jack and Julia the Tet is an alien artificial intelligence. Determined to acquire Earth’s natural resources and wipe out humanity, it destroyed the Moon and then used an army of Jack clones and drones to invade Earth. The story of the scavenger invasion and humanity’s move to Titan is a Tet fiction. Jack repairs the captured drone and prepares to launch it, but other drones attack the base. Although the humans fend off the attack, it leaves Beech gravely injured and the captured drone damaged beyond repair.
With no other choice, Jack volunteers to fly the nuclear device to the Tet and use his delivery of the ‘hibernating’ Julia to explain his presence. During the flight, Jack listens to the Odyssey’s flight recorder and learns the truth: he was the commander of a NASA mission to explore Titan, Vika was his co-pilot, and Sally was their mission director on Earth. When NASA discovered the Tet, they diverted the Odyssey to investigate. When the Tet drew in their ship, Jack jettisoned Odyssey’s sleep module that held Julia and the rest of the crew to allow it to return to Earth.
In the present, Jack enters the Tet and finds thousands of chambers that each contain clones of himself and Vika in stasis. He opens the stasis chamber to reveal the wounded Beech and a bomb made from the Odyssey’s fuel cell. They detonate it, killing themselves and destroying the Tet. On Earth, Julia witnesses the explosion from the lake house. Three years later, she and her young daughter meet the surviving resistance members accompanied by Tech 52, whose true memories were triggered by his encounter with Julia and Jack.