The Avalon, a sleeper ship transporting 5,000 colonists and 258 crew members in hibernation pods, is on course to the planet Homestead II, a journey lasting 120 years. Thirty years into the journey, an asteroid collision damages the ship and causes its computer to awaken one passenger, mechanical engineer Jim Preston, a person who "fixes things", 90 years too early.

After a year of isolation, with only an android barman named Arthur for company, Jim grows despondent and contemplates suicide. One day, he notices a beautiful young woman named Aurora Lane inside her pod. Jim views Aurora's video file and is smitten by her humorous personality. After struggling with the morality of manually reviving Aurora for companionship, therein robbing her of a life before arriving to a new planet, he awakens her, letting her assume her pod also malfunctioned. Jim then asks Arthur to keep the secret that he woke her up. Aurora, devastated at having to live out her life on the ship, unsuccessfully attempts to re-enter hibernation. Resigned to the situation, Aurora, a journalist, begins writing a book about her experiences.

Over the next year, Jim and Aurora grow closer, eventually falling in love. Jim intends to propose to Aurora, but Arthur inadvertently reveals the truth to her. Aurora, anguished, alternately berates, shuns, and physically attacks Jim. She furiously rejects Jim's pleas for forgiveness, and the two avoid contact.

Soon after, another pod failure awakens Gus Mancuso, Chief Deck Officer. He discovers multiple failures throughout the ship's systems. If not repaired, the ship will continue suffering critical system failures and the mission may fail. Gus attempts to repair the ship with Jim and Aurora's help but he soon falls critically ill, having been physically damaged by his malfunctioning pod. The Autodoc, the ship's automated medical diagnostics and treatment pod, reveals Gus has a prognosis of only hours to live. Before dying, Gus gives Jim and Aurora his ID badge to access crew-only areas and to repair the ship.

Jim and Aurora discover a series of hull breaches from the asteroid collision two years earlier. The computer module administering the fusion reactor powering the ship has been critically damaged, causing the ship's cascading malfunctions. Jim and Aurora replace the damaged module. When the computer fails to vent the reactor to extinguish a massive reactor fire, Jim has to vent it by opening the vent hatch from the ship's exterior while Aurora assists from inside. She admits she is terrified of losing Jim and being left alone. After venting the reactor, Jim's tether snaps and his damaged spacesuit loses oxygen; Aurora retrieves Jim but has to resuscitate him in the Autodoc.

Afterwards, Jim learns that the Autodoc can function as a makeshift hibernation pod for one person. He can put Aurora to sleep for the remainder of the voyage.

Eighty-eight years later, the ship's crew awaken on schedule, shortly before arrival at Homestead II. They discover a small house amid lush flora and fauna in the ship's grand concourse area. Aurora's book reveals that she chose to stay awake with Jim and live out the rest of her life with him on the Avalon as she continued writing her story.

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