In a living room playing chess, Adam Gherrity and Jane Kuang debate whether NASA would spend billions to save one astronaut stranded on Mars. The chat nods to The Martian, but Adam pushes beyond, wondering how such a rescue would play out in real life. Saturn glows directly outside their window, a stark reminder of the vast distance. Jane concedes that Megan Benson might share Adam's optimism, but he notes she went crazy, and nearby lurks the remains of another astronaut.
In their third year aboard their isolated ship, the crew follows a careful routine: joint exercise, then daily tasks. Jane takes charge of spacecraft maintenance and navigation, while Adam tends to their plants, the chickens, and a goat. They fill the long days with chess, films, and music, trying to make the confinement bearable. After a cardio dance session, Jane chides Adam for masturbating in the greenhouse, and he proposes they have sex as a stress-reliever without romance. Jane laughs, but the discussion reveals competing motives: she lists three reasons not to have sex—No Attraction and Both Married are two of them, and the third, Space Babies, remains unspoken at first.
Together they watch Alien, though Adam resists at first. The film unsettles him, and he ends up crawling into Jane's bunk. Half asleep, she forgets she is unclothed and wakes pressed against him. She suggests cuddling could benefit their mental health, but the heated moment brings up the third reason not to have sex: Space Babies. With both of them married to others back home, condoms are not provided. Adam jokes about radiation potentially making Jane sterile, and she responds with disgust as she leaves to gather her thoughts.
They vent about one another to the inanimate Benson, and Jane confesses she has turned four power drills into dildos, admits she’s contemplated sex with him, and acknowledges she finds him attractive. Adam dismisses her reasons as ridiculous, knowing that rescue seems unlikely. They watch video messages from their spouses; Adam hopes his political-aspiring wife Savannah hasn’t waited for him, while Jane fears Tom would have loved being with her. Still, they lean on each other and grow closer, sharing more and more moments together. When Adam runs dry, Jane makes him homemade pop tarts. After the goat dies, they lose fresh milk but gain fifty meals, a small silver lining in their harsh reality.
After agreeing to do “everything but” sexually, they regain navigational control and calculate a three-week journey home. Yet their resolve dissolves, and they resume sex with increasing frequency. They reflect on what they miss: for Jane, exploration and music; for Adam, his wife's hugs. He hopes to stay best friends, but she resists that idea.
Back on Earth, a press conference reveals the strain in their marriages. The memory of Benson resurfaces as Jane falsely claims she died in her sleep, though a flashback shows her having to axe her to save Adam when she briefly lost her mind. Savannah appears, kisses Adam, and leads him away, ushering them into a new round of medical tests. Tom reveals that he had dated around, but nothing serious, while Savannah has climbed the political ladder from mayor to governor.
In their debriefings, Adam talks about experimenting with crossbreeding blueberries and marijuana. Jane describes Benson turning violent and demonstrates the drills they adapted. With their spouses completing discharge papers, Jane and Adam say their goodbyes. He asks to stay in contact, but she refuses.
Back in Savannah's world, Adam feels invisible. He has kept the shuttle chickens in their house, where they buzz and bother Savannah. Jane and Tom talk in bed, and she confesses that dancing with Adam was her favorite part of space life. Tom initially resists a trip, but relents when reminded that Jane had been cooped up for years.
During a run, Jane hears a song that reawakens her feelings for Adam. At home, she discovers Tom had been seeing Amber, prompting her to insist he contact her. She also confesses her and Adam's sexual relationship. Nine days after their return, during a White House dinner, Jane tells Adam and Savannah she split from Tom. The President then presses them to showcase their dancing skills, revealing the visible chemistry between Adam and Savannah; that moment helps drive a wedge between Adam and Savannah, leading to a breakup.
In the end, Adam seeks out Jane and declares they are meant to be together, choosing to take a trip away from everything else.
