The starship Avalon is transporting over 5,000 colonists to the planet Homestead II, a journey that takes 120 years. The colonists and the entire crew are in hibernation pods, but a malfunction awakens one passenger, mechanical engineer Jim Preston (Chris Pratt), 90 years early.
After a year of isolation, with automated food and entertainment systems available but no company except Arthur (Michael Sheen), an android bartender, an increasingly despondent Jim notices beautiful Aurora Lane (Jennifer Lawrence) in one of the pods. Her video profile reveals a humorous personality and that she is a writer. Jim struggles over the morality of manually reviving Aurora for companionship, but eventually does. He lets her believe that her early awakening was a hibernation pod malfunction like his. Aurora, devastated that she may grow old and die before the ship reaches Homestead II, attempts a fruitless effort at re-entering hibernation, just as Jim once tried and failed to do. She comes to accept her situation and begins writing a book about her experiences. Jim and Aurora grow closer, becoming lovers.
A year later, Aurora accidentally learns the truth when Arthur inadvertently comments on how difficult it was for Jim prior to Aurora's awakening. She is distraught, alternately berating, shunning, and physically attacking Jim. Soon after, however, another pod failure awakens Gus (Laurence Fishburne), a Chief Deck Officer. The three discover multiple failures in the ship's systems due to a failed component overloading other components. Gus attempts repairs with Jim and Aurora's help, while Aurora still blames Jim for stealing her life, claiming it is tantamount to murder. Gus' body begins failing; medical tests in the Autodoc, an automated medical diagnostics and treatment pod, show that his hibernation pod's malfunction has left him with hours to live. Before his death, Gus gives Jim and Aurora his ID badge to access crew areas and repair the ship, which is experiencing more severe failures each day.
The two trace a colliding asteroid's path through the ship's hull from two years earlier that damaged the computer administering the reactor. Fixing the computer leads to further reactor damage. Jim realizes he must vent a fusion reactor by opening an outer vent hatch. Aurora assists Jim while admitting she is terrified of losing him and living on the ship alone. Aurora, from inside the ship, and Jim, outside it, successfully vent the reactor. However, Jim's tether breaks loose and his damaged spacesuit loses its oxygen supply. Aurora retrieves the dead Jim and resuscitates him using the Autodoc. Jim later learns that with crew access privileges the Autodoc can re-hibernate one person. He offers Aurora the choice of going back to sleep, and she realises this choice would mean that she would never see him again.
After 88 years, the ship's passengers and crew awaken shortly before arrival on Homestead II. They discover vegetation and domestic animals in the ship's concourse area. Aurora's completed book is found, and it is revealed that she chose to stay awake with Jim and finish writing her story.